"Barring anything really crazy happening, today will certainly be one for the sprinters. There will be a lot of teams looking to set it up for a bunch sprint, especially if they've not had the chance for one so far. In terms of contenders there's obviously the pure sprinters like Mark Cavendish, Andre Greipel, Alexander Kristoff and Nacer Bouhanni."
Post update
Right then, that's stage four done and dusted. the race report is taking shape so it's time to move on to stage five...
Martin in yellow
Yellow Jersey
Classification
1. Tony Martin (Ger) Etixx - Quick-Step 12hrs 40mins 26secs
2. Chris Froome (GB) Team Sky +12secs
3. Tejay van Garderen (US) BMC Racing +25secs
4. Tony Gallopin (Fra) Lotto +38secs
5. Peter Sagan (Svk) Tinkoff - Saxo +39secs
6. Greg Van Avermaet (Bel) BMC Racing +40secs
7. Rigoberto Uran (Col) Etixx - Quick-Step +46secs
1. Tony Martin (Ger) Etixx - Quick-Step 5hrs 28mins 58secs
2. John Degenkolb (Ger) Giant +3secs
3. Peter Sagan (Svk) Tinkoff - Saxo
4. Greg Van Avermaet (Bel) BMC Racing
5. Edvald Boasson Hagen (Nor) Team MTN
6. Nacer Bouhanni (Fra) Cofidis
7. Jacopo Guarnieri (Ita) Katusha
8. Tony Gallopin (Fra) Lotto
9. Zdenek Stybar (Cze) Etixx - Quick-Step
10. Bryan Coquard (Fra) Europcar
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Jess Parsons: CRACKING end to today's stage on #TDF2015 TT specialist @tonymartin85 takes it! Moments like that are what cycling is all about.
Stuart Mitchell: Sky won't care, they wanted to get through the day, and it's only 12 seconds. Froome will reclaim yellow in a few days.
Andrew Harvey: I'm sweating just sitting at my desk reading the updates! Fantastic effort from @TeamSky and stage winner @tonymartin85! #bbccycling
Post update
Rob Hayles
Former GB cyclist on BBC Radio 5 live sports extra
"Today shows why Dave Brailsford set the team set up around Chris Froome with a Classics orientated squad. It was particularly for this stage. This was the key one in the opening week where they could have lost significant time but they've come up trumps.
"I'm glad there were no big losses today though because it's kept everyone in the chase, the top four in particular. It's shaping up to be an incredible race."
Post update
I must admit to missing that bit. I saw Martin suffer a problem with around 20km remaining. It appears as though he got on Matteo Trentin's bike and rode to the finish on that.
And for those of you wondering why Team Sky and Chris Froome will be happy to be rid of the yellow jersey - it's mainly to do with the pressure of holding it. They will now be able to relax over the next few days and worry about winning it back when they hit the Pyrenees next week.
Post update
Here's Tony Martin's thoughts on that finish: "After having a flat tyre and changing bikes I was thinking about the next stage. But I got back in and with 5km remaining everybody was looking round and not pulling so I went for it. A thousand thanks for my team. I am super happy."
Post update
Big hugs and kisses all round in the Etixx - Quick-Step team. It's been an eventful few days for Mark Cavendish and his team-mates. After messing up the sprint on stage two, they got it so, so right today.
The top four
Martin's German team-mate John Degenkolb was second over the line, one second adrift, with Peter Sagan third and Greg van Avermaert fourth.
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Rob Hayles
Former GB cyclist on BBC Radio 5 live sports extra
"What an incredible finale for Tony Martin. He has just missed out on the first three stages. He deserves it so much. He started out just one second behind Chris Froome and Team Sky will not be bothered to lose the yellow jersey."
Martin wins and takes yellow
Yellow Jersey
Classification
APCopyright: AP
Tony Martin wins stage four and takes the yellow jersey.
Post update
Martin is into the final 200m, he's going to win this and ride into the yellow jersey...
1km remaining
Mark Cavendish is hovering in the train, about eighth wheel. He does not need to worry about a leadout because his team-mate Martin is going for a solo victory. But Martin is only five seconds clear as he enters the final kilometre.
2km remaining
Tony Martin is a three-time time trial world champion so he is used to putting in a big effort and holding on. He has 2km to go. If he wins, he is in the yellow jersey.
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Rob Hayles
Former GB cyclist on BBC Radio 5 live sports extra
"I don't think Team Sky will be chasing. They won't mind if they lose the yellow jersey today."
Martin attacks
There are two right-angled bends in the final kilometre and the last 600m is uphill. Doesn't sound perfect for Cav, who prefers the flatter sprint.
And while everyone is looking at each other, Tony Martin goes on the attack.
Post update
Rob Hayles
Former GB cyclist on BBC Radio 5 live sports extra
AFPCopyright: AFP
"Mark Cavendish is in this lead group. On this finish if he's got the legs, he could be getting his hands in the air. But his Etixx - Quick-Step team have got Tony Martin to think about as well."
5km remaining
We are 5km from the finish and all the big favourites are in the mix. As exciting as the cobbles have been, they've not been quite as divisive as last year.
I don't think any of the big four will be looking for a stage win. So it's down to the likes of Sagan, Degenkolb, Van Avermaert etc...who's your money on?
Geraint Thomas and Chris Froome are back on the front of the bunch. I want what G has for breakfast.
Group back together
EPACopyright: EPA
Incredible scenes on the road. Froome, Nibali and Valverde are in this leading group. Peter Sagan has been distanced. Can they keep the speedy Slovakian adrift?
Geraint Thomas, refreshed from taking a quick break, takes up the pace again but the group behind is catching them.
10km remaining
There is a group of around 10 riders clear of the bigger group. Geraint Thomas finally sits up for a breather, leaving Froome out front. Froome flicks an elbow, indicating he wants someone else to take over.
Attacks galore
Nibali goes first but he is passed by Zdenek Stybar. Froome goes into Nibali's wheel and then goes past the Italian.
Geraint Thomas chases down Stybar and then looks over his shoulder and waits for Froome and the two of them emerge from the cobbles right at the front. Terrific riding by G.
Post update
Will anyone make a dash for it on this final section of pave? Astana again take up the pace, but once more Geraint Thomas is up at fourth wheel and Chris Froome slips in behind him. There are about 40 riders strung out behind. Vincenzo Nibali sidles up alongside Froome. Alberto Contador and Nairo Quintana are still in the leading group.
Post update
Rob Hayles
Former GB cyclist on BBC Radio 5 live sports extra
"Team Sky have played a blinder so far."
15km remaining
The riders all look like they've been down a coal mine as they enter the final 15km to the finish. Just one cobble section to come. It's 2.3km long and there will be 7km remaining when they get back on the asphalt.
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Matt Slater
BBC Sport at the Tour de France
ReutersCopyright: Reuters
"As expected, it has all come to life on the cobbled run-in to Cambrai. Chris Froome's rivals have all had a go at trying to put him under pressure but the Team Sky rider's wobbliest moment so far has been when he rode into the gutter on a relatively decent stretch of road.
"It is the French favourites who have been having the hardest time of it, which is ironic given the fact that we are actually in France now.
"Pierre Rolland was dropped by the leaders' pace relatively early but Thibaut Pinot has really been in the wars and was last seen slamming his bike onto the road in frustration. He badly needs a new steed."
Martin mechanical
Meanwhile, the leaders are off the cobbles. Just one section of Pave remaining. Simon Brotherton reports that Tony Martin is out the back. Unclear whether a puncture or a mechanical for the German. He is just one second behind Chris Froome in the overall race standings remember.
Pinot is off his bike again and he slams it into the side of the road. Frustrations boiling over for the Frenchman. He is three minutes behind the leaders now.
Post update
Rob Hayles
Former GB cyclist on BBC Radio 5 live sports extra
"I wouldn't imagine it will be bon appetit for the FDJ team when Pinot turns up for tea tonight."
Pinot punctures
The choking dust will not be helping the riders. At the back of the leading bunch, France's big hope for this year's Tour, Thibaut Pinot, comes to a halt. Looks like a puncture at the worst possible time. Pinot is looking distraught as he gets a shove from his mechanic.
Post update
There are about 30 riders in a long line cycling right up the middle of the road. Tinkoff-Saxo put a man on the front. Geraint Thomas is around 12th with Chris Froome still hugging his wheel as they head though the fields of northern France.
Post update
It looks like Geraint Thomas is last-Skyman standing with Chris Froome as they get back on to the tarmac. Thankfully no rain and no fallers so far but this is punishing racing as the riders swing right on the penultimate section of cobbles.
25km remaining
Getty ImagesCopyright: Getty Images
Here we go on the cobbles again. Vincenzo Nibali attacks again but almost rides into the back of Tony Gallopin. The Italian gives the Frenchman a shove in the backside.
John Degenkolb takes up the pace with Geraint Thomas hanging on in there, with Chris Froome following his every move.
Froome almost falls
Having just typed that, Froome comes within a whisker of being shovelled off his bike. That was close. It was a Katusha rider that nudged him wide and he clipped the kerb before being bounced back into the peloton.
Froome in position A
Geraint Thomas and Chris Froome momentarily drop back into the middle of the leading bunch but they drift out to the right and into open road and from a precarious position, they are in position A again.
Pave sector three coming up. It's 1500m long and followed quite quickly by pave two which is 3.7km.
30km remaining
Lotto Jumbo, who have classics specialist Sep Vanmarcke in their midst, storm to the front of the peloton but Nibali is having none of that and puts his Astana boys back up there.
Post update
BMC Racing have several riders in the front group, riding for Greg van Avermaert. Geraint Thomas is continuing to show Chris Froome the way. There are a lot of nervous glances around. The second, much larger, bunch on the road is around 35 seconds adrift.
Kristoff punctures
The riders are strung out again. Bad news for Alexander Kristoff who has suffered a puncture. That could end his hopes of a stage win. Andre Greipel, the leader of the points competition, is also losing ground.
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Small Man Peaky: This is insanity. Froome is going to need Stannard and G to get him to the safety. I hope Martin gets the yellow. #bbccycling
35km remaining
BMC Racing's Greg van Avermaert is looking interested, as is Movistar's Alejandro Valverde. There are about 40-50 riders left in the front group as they career on to pave sector four.
Post update
Getty ImagesCopyright: Getty Images
They are off the cobbles and this is like the finish to a one-day classic. No sizeable gaps opening up. Zdenek Stybar of Etixx - Quick-Step takes up the pace and he is joined by team-mate Matteo Trentin, while Mark Cavendish is also up there. Will they be successful in getting Tony Martin into yellow?
I can see the white sunglasses of Geraint Thomas in about fourth wheel.
Nibali attacks
The breakaway is swallowed up and Vincenzo Nibali immediately goes on the attack with his Astana team-mates. Ian Stannard and Geraint Thomas respond. And Chris Froome muscles his way into sixth place in the line.
More cobbles
Getty ImagesCopyright: Getty Images
Team Sky's Ian Stannard swings across in front of an Astana rider to take on the peloton pace-setting. Chris Froome, like he is on a leash, is inches behind. Geraint Thomas also up at the front. The Welshman loves the cobbles and is the perfect man to chaperone Froome as they enter sector five of the pave.
Post update
All of the big boys appear to be together though. We are 5km from the next section, which is 1.6km long.
Nervy times on the cobbles
EPACopyright: EPA
Team Sky are alert to the danger and close it down and Nibali sits up. Chris Froome is in the wheel of Ian Stannard and Geraint Thomas is close by. Nervy times for the big general classification riders.
Post update
Team Sky have got helpers every 500m along the sections of pave.. Etixx - Quick-Step are leading the way. Tony Martin is the man on the front. Vincenzo Nibali is behind him and as they come off the cobbles, his Astana team-mate gives him a nudge and off they go.
The break almost done
It's reached that time of day where the breakaway is going to be swallowed up, so it's time for a final namecheck as they enter the second of today's seven cobbled sections.
The men in the break are: Lieuwe Westra (Astana), Thomas De Gendt (Lotto-Soudal) who initiated the breakaway, Perrig Quemeneur (Europcar) and Frederic Brun (Bretagne-Seche Environnement).
There are clouds of dust being whipped up by the motorbikes and cars.
BBC Sport's Gemma Sterba is with Rob and Si in the commentary booth. She's just emailed in to tell me: "Just been on the coffee (and cookie run) for the boys. Few rain drops... couple of brollies going up. Wish I hadn't left my jacket in the car now."
Dowsett down
There has been a crash or two in the peloton. Nothing major. A slightly damp road and a tight right-hander to blame. It was a low-pace crash and there are no casualties.
At the back Britain's Movistar rider Alex Dowsett is down and he has a bloodied elbow. He will get back on though. The leaders are just one minute clear as Vincenzo Nibali's Astana team up the pace.
50km remaining
Getty ImagesCopyright: Getty Images
The list of potential winners today is long. Peter Sagan, John Degenkolb, Greg van Avarmaert, Tony Gallopin, Tony Martin.
And in the bigger picture, will Vincenzo Nibali be able to get any time back on race leader Chris Froome? How will Alberto Contador and Nairo Quintana fare? We are about to find out folks.
55km remaining
Serious race heads on in the peloton. Andre Greipel, in his bright green points leader jersey decides the pace is not high enough, and he stomps on his pedals. He is looking to repay the help he has been given so far in the race - maybe Tony Gallopin is the man he is working for.
Here comes the rain
Simon Brotherton
Cycling commentator on BBC Radio 5 live sports extra
"One or two spits and spots of rain landing on the window."
The cobbles are coming
Getty ImagesCopyright: Getty Images
The pace is being lifted in the peloton and all the teams are organising their trains. Team Sky are hugging the left-hand side of the road. Alberto Contador's Tinkoff-Saxo boys are down the middle with Nairo Quintana and Alejandro Valverde's Movistar.Kristoff's Katusha down the right. Just 10km to the cobbles.
60km remaining
Rob Hayles reminds us that the general calculation for catching a break is one minute for every 10km. So the four escapees, who are three minutes clear, have little chance of making it to the finish. Although, if the cobbles get damp...
Post update
Now, from a neutral point of view, a bit of wet would spice things up nicely for the finish. I'm not suggesting I want anybody to come off and get injured but a couple of slips and slides would liven up the final 50km.
I can guarantee none of the riders will want it. The sky is slate grey though...
Post update
Simon Brotherton
Cycling commentator on BBC Radio 5 live sports extra
"Looking up to the skies, I'd say it's 50-50 as to whether it rains at the finish before the riders get here."
Rob Hayles adds: "It's a big game changer if it does rain. It makes the cobbles extremely slippery."
In the break
The leading quartet are continuing along their merry way, tapping out a decent pace. Their lead is still around three minutes as they approach 65km to go. Just 20km to the next section of pave.
Post update
Rob Hayles
Former GB cyclist on BBC Radio 5 live sports extra
Rob found a bit of coverage from the big crash on yesterday's stage, seen through the eyes of a Lotto NL Jumbo mechanic. It's an interesting insight into life at the Tour.
Jess Parsons (14:43) will no doubt be pleased to hear that 'Big Si' is up and running on BBC Radio 5 live sports extra. Mr Brotherton is joined in the commentary booth by Rob Hayles. Click on the link above to listen in.
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Ieuan Llyr Jones: (re Elliot 14:27): Big Mig did. 1992. Won the prologue, lost next day. Won it back 11 days later. #bbccycling
Mark Williams: In addition to Nibali last year, also Lance Armstrong in 1999, 2002, 2004 & 2005. Not that they count any more!
Big Mig is five-time Tour winner Miguel Indurain, the only rider (since Lance Armstrong's seven on the trot were deleted from the record books) to win five successive races. The Spaniard was dominant from 1991-95.
Why it's good to be at the front
The race is calm again. But we are only 30km from the cobbles. And that means the action will start in around 20km as the peloton picks up the pace to put their riders in position.
Geraint Thomas told BBC Sport before the race that being up near the front is good for several reasons. Firstly, the roads narrow so there's less space. Secondly, you are less likely to be involved in, or caught behind and therefore delayed by a crash.
The other bonus for Team Sky is that because they have Chris Froome in yellow, their team car is first in the line of 22 that follow the race. Therefore, if Froome crashes, or punctures, his car will be with him the quickest.
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Matt Lewis: (re Elliott 14:27) Vincenzo Nibali, last year? Got it stage 2, lost it stage 9, regained it on stage 10 and kept it till the end.
Jess Parsons: Peter, loving the live text but there's nothing like Big Si getting me animated all afternoon whilst I sit at my desk! #bbccycling
Glad to be the warm-up act Jess. Not long to wait for 'Big Si' and Rob Hayles. Live coverage on BBC Radio 5 live sport extra from 15:00 BST. Buttons to push at the top of this page.
Sagan suggests an attack
A nice touch from Peter Sagan there. He follows Cavendish, Greipel and Degenkolb over the line and while they have been battling for points, the peloton has been left a couple of hundred metres behind. Sagan, with a smile, gesticulates that they should go off on the attack.
His cheeky request is met with stony looks. He shrugs, laughs and they all drop back to the comfort of the bunch.
Cav wins minor sprint place
Green Jersey
Classification
And it is Cavendish that takes the 11 points. Behind him Greipel and John Degenkolb have a little coming together as they battle to get on the wheel of Cav. Six of one and half a dozen of the other but it knocks Greipel off his stride and he will be seventh over the line.
Post update
Green Jersey
Classification
Thomas de Gendt of Lotto-Soudal suddenly takes off and he takes the 20 points (and 1500 euros prize money). De Gendt is a team-mate of Andre Greipel and he is two minutes behind and in the middle of his sprint. Mark Cavenidish is in the shadow of his lead-out man Mark Renshaw....
Post update
Green Jersey
Classification
Up the road the four in the break are not looking too interested as they edge into the outskirts of Havay. Astana's Lieuwe Westra is doing the least work, he has bigger battles to come when his team leader Vincenzo Nibali decides to make use of him.
Post update
Green Jersey
Classification
The intermediate sprint in Havay marks this year's entry into France. The sprinters are lining themselves up in the peloton...
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Elliott Riddle: Has any rider ever secured yellow this early on, subsequently lost it, and then regained it again come the Champs-Elysees? #bbccycling
Elliott, I'm sure one of your fellow readers will supply you with an answer...
Sprint coming up
Green Jersey
Classification
It's all rather quiet out on the road and that, given the carnage of yesterday and the chaos to come, is probably no bad thing.
The leaders are approaching the intermediate sprint in Havay. The four in the break will get 20, 17, 15 and 13 points. The first man from the peloton will get 11. Expect Mark Cavendish, Andre Greipel, who leads the classification, and Peter Sagan to be interested.
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Tom Oxley: Cobbles is doing the pave a disservice, these things are the size of a baby's head.
Ken Smith: Hoping Tony Martin can pinch some bonus seconds & therefore yellow. Sky don't want it now! G is in top form, go G (that's Geraint Thomas).
Amber Gahan: It's that time of year again when I am glued to #bbccycling text commentary of Le Tour at work
In the break
Lieuwe Westra, the Astana team-mate of Vincenzo Nibali, drops back to his team car and hops off his bike. He too is changing for a more cobbles-friendly racer.
He is of course in the break and the quartet's lead is hovering around the the three minute mark with 100km, or about 60 miles if your prefer, of today's stage remaining.
Panic attacks for Matthews
Getty ImagesCopyright: Getty Images
One rider not looking forward to the cobbles is Michael Matthews. The Orica GreenEdge rider went down and took a couple of layers of skin off, prompting Rob Hayles to say on commentary: "Matthews looks like he may well have lost some of his tattoo on his back. That will take some touching up in the coming months."
Matthews says: "My goal is just to get to the cobbles and see how it goes and make the time cut basically. Last night I was really struggling to breathe. That wasn't great. I had little panic attacks I could not breathe, it was quite scary."
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Andrew Harvey: Favourite's for this stage have to be Kristoff and Degenkolb. Both had quality classics campaigns and should finish strongly. If Stannard and Thomas can get Froome safely to the end then he should stay in Yellow. Whether he wants to is another matter. #bbccycling
In the break
The pace has been knocked off by the chasing peloton after that burst of speed into the cobbles. That means our four plucky riders out front are extending their advantage again and are now more than three minutes clear.
They are: Lieuwe Westra (Astana), Thomas De Gendt (Lotto-Soudal) who initiated the breakaway, Perrig Quemeneur (Europcar) and Frederic Brun (Bretagne-Seche Environnement).
Bike changes
A couple of riders have changed their bikes after that opening section of cobbles, also known as pave. Alberto Contador was one of them.
It was a planned move and he will have slightly wider tyres and better suspension to absorb the bouncing around on the cobbles. Team Sky are also on specialist bikes for today's stage. It's all about marginal gains.
The riders have 110km remaining - the next section of pave comes 46km from the finish.
Crestfallen Cancellara
Getty ImagesCopyright: Getty Images
Fabian Cancellara has taken to Twitter as he contemplates the end of his 2015 Tour with a fractured back. The rider who goes by the nickname Spartacus, wrote: "One day you win one day you lose but I will not give up on what happen #spartacus @TrekFactory thanks for all your messages and comments.
"On the way home with lot's of pain all over my body. Doesn't feel nice to crash at 75km/h. This morning it felt horrible to stand out from bed."
Post update
Peter ScrivenerCopyright: Peter Scrivener
Here's a slightly closer look at the finish that awaits our riders in about two and-a-half hours..
The BBC Radio 5 live team have stolen a march on the peloton. They're at the finish line in Cambrai. Guess what - there's cobbles.
You can listen to commentary from 15:00 BST on BBC Radio 5 live sports extra and on this page.
Post update
Mark Cavendish is on the front of the peloton. He will be working for his Etixx - Quick-Step team-mate Tony Martin, who is second behind Chris Froome - one second adrift.
The Manx Missile is burying himself here to help his team-mate. Cavendish was clearly gutted to miss out on winning stage two but that is a long way behind him now.
The first section of pave is safely negotiated. An army of team mechanics wait at the end with tyres held high just in case any of their riders have punctured. But all seem safe and on we go.
Cobbles section one
Here we go on to the cobbles. They look pretty smooth to me, in comparison to some of the sections in Paris-Roubaix. The leaders enter the first of seven sections 70- seconds ahead of the peloton.
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So, who are you backing for the stage win today? Who will cope best with the cobbles? Will Froome hold on to the yellow jersey?
BBC Radio 5 live sports extra commentator Simon Brotherton has gone for: John Degonkolb to win, Greg van Avermaert to take the yellow jersey.
Matt Slater has gone for the same as Simon.
Rob Hayles is backing Tony Gallopin for the stage win and can't make up his mind between Van Avermaert and Peter Sagan for the yellow jersey.
You can hear live commentary of the closing stages from 15:00 BST by clicking the relevant link at the top of this page.
Text me your thoughts on 81111. Or tweet #bbccycling
Post update
Matt Slater
BBC Sport at the Tour de France
"Some day soon we are going to get a nice, routine, first-week-of-the-Tour stage and we can all take a long lunch and change for dinner. It won't be today, though.
"The longest stage, seven sections of cobbles, wind, a forecast of rain and the certainty of sustained aggression.
"With Froome in yellow, earlier than expected, his rivals must stop Team Sky from gathering any more momentum.
"Nairo Quintana simply has to stop losing chunks of time, Alberto Contador could use a relative win and Vincenzo Nibali needs a repeat of last year's virtuoso demonstration of strength and skill.
"The Italian already has team-mate Lieuwe Westra up the road. The foundations for an attack have been laid."
In the break
Four riders broke clear from the peloton in the early knockings of today's stage and were allowed to build up a lead of approaching 10 minutes but they have been pulled back in to a more manageable two minutes.
The men in the break are: Lieuwe Westra (Astana), Thomas De Gendt (Lotto-Soudal) who initiated the breakaway, Perrig Quemeneur (Europcar) and Frederic Brun (Bretagne-Seche Environnement).
Westra is the interesting one because he is a team-mate of one of Froome's main rivals Vincenzo Nibali - and the Italian is decent over the cobbles.
Here come the cobbles
The riders are already off and pedalling today and have completed around 100km of the 223km route. They are approaching the first of the seven cobbled sections and the pace is high.
True to Geraint Thomas's prediction, it's all about being first into the cobbles and the Team Sky train is right on the front of the peloton. They have around 5km to the first section. The remaining six all come in the final 50km
Geraint's guide to stage four
Tour de FranceCopyright: Tour de France
"Last year Team Sky didn't have the best of days on the cobbles, when Froome was forced to abandon, but personally I did pretty well. It's definitely a stressful day, but I enjoyed it and I'm looking forward to it again.
"If it's dry it won't be as bad, but if it rains like last year there could be big time gaps again. It's certainly a big day for the teams with riders competing for the overall victory.
"Everybody will be stressing about that and you've just got to concentrate on being in a good position at the front. The only problem is that's what 180 other riders want to do. It's all about fighting for position for when most of the cobbled sections start in the final 50km or so."
Cobble time
It is the longest stage of this year's race. A 223.5km (138.8 miles) ride from Seraing to Cambrai as the race leaves Belgium and crosses into France for the first time.
There are no significant climbs to negotiate today but what we do have is cobbles. Almost 13.5km spread across seven sections. If you've watched Paris-Roubaix and the one-day Spring Classics you'll be well aware of the chaos that the cobbles can cause.
If you're new to all this, you are in for a real treat today. Check out what Britain's Team Sky rider Geraint Thomas told us in his Tour de France stage-by-stage guide...
Post update
Marginal gains has been one of British Cycling saviour Sir Dave Brailsford's mantras over the years and it certainly paid dividends on stage three. But enough of stage three. That was so yesterday.
Today is all about stage four...
Marginal gains
Peter ScrivenerCopyright: Peter Scrivener
Maybe Froome should have held back by 0.07 seconds. If he had been awarded a time one second behind stage three winner Joaquim Rodriguez, instead of the same time, Germany's Tony Martin would be in the yellow jersey today.
Post update
Rob Hayles
Former GB cyclist on BBC Radio 5 live sports extra
"Team Sky rode extremely well. Chris Froome, when he went with around 500m to go was very early but he set himself up to finish ahead of his main rivals and Alberto Contador was the big loser (finishing 18 seconds adrift of Froome).
"It's very early in the race for Froome and his team-mates to defend the yellow jersey. They have probably got themselves in this position without wanting it."
Also on the podcast are our new race leader Chris Froome and his Team Sky boss Dave Brailsford.
Froome admits he is "surprised" to be in yellow and that "it's really early days".
Brailsford said getting the race lead was reward for "fantastic effort".
He added: "It's a big deal getting the yellow jersey, the most famous jersey in cycling and every time you get the opportunity to wear it you should respect it and be super proud of having it in the team and I know that's what we will be."
Matt White, sporting director at Orica GreenEdge, told BBC Radio 5 live Sport Extra that it was "a fair decision" to stop the race. Several of his riders were caught up in the crash and he lost Simon Gerrans and Daryl Impey from his nine-man squad.
Britain's Simon Yates, who also rides for the Australian-based outfit, went down in the crash but he recovered to finish eighth on the stage.
"Rider safety comes first," said White. "These rules have to be taken on merit from crash-to-crash."
The race was stopped for a little over 10 minutes to allow race doctors to attend to the injured, which caused a little confusion among those still riding.
Team Sky's Geraint Thomas said that he and a lot of the riders were not totally sure why the race had been stopped until they reached the finish.
But most people were in agreement that race director Christian Prudhomme had made the right decision in calling a halt to proceedings.
Prudhomme defended his actions, saying that although the main crash was "more spectacular than serious", the four ambulances and two doctors cars that follow the race were needed to treat the injured.
He added that he had no option but to stop the stage because "you must have medical assistance vehicles behind the race".
Cancellara & Impey abandon
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Switzerland's Fabian Cancellara was the most high-profile casualty. It turns out the race leader, who was catapulted head first into a ditch, finished the stage despite fracturing two vertebrae in his back.
South Africa's Darly Impey was another to call it a day - although he rode the final 60km or so with a dislocated collarbone.
Australia's Simon Gerrans, Dutchman Tom Dumoulin, Dmitry Kozontchuk of Russia and France's William Bonnet, who sparked the chaos by being the first to come off his bike, are also out.
Andreas Schillinger of the Bora-Argon 18 team was also a non-starter this morning.
Tour carnage
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Hello and welcome to live text and radio commentary (from 15:00 BST) of stage four of the 2015 Tour de France. It was another crazy Tour day on Monday with crashes, broken bones, dislocations, retirements and a tremendous finish up the impossibly steep Mur de Huy.
I'll move on to stage four after a brief look back at the carnage of day three which resulted in seven abandonments.
Froome in yellow
Yellow Jersey
Classification
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"I would much rather be in this position than having to make up time on my rivals," said British rider Chris Froome after surprising himself by taking the overall lead of the Tour de France on Monday's hectic third stage.
Live Reporting
Peter Scrivener
All times stated are UK
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Latest PostPost update
That's all from me today. Thanks for your tweets and company. I'm back on Friday to take you through the weekend's action.
Paul Fletcher will be in the chair for Wednesday's stage. Be sure to join him.
Geraint's stage five preview
It's a 189.5km (117.7 miles) race from Arras to Amiens.
Here's what Geraint Thomas is expecting:
"Barring anything really crazy happening, today will certainly be one for the sprinters. There will be a lot of teams looking to set it up for a bunch sprint, especially if they've not had the chance for one so far. In terms of contenders there's obviously the pure sprinters like Mark Cavendish, Andre Greipel, Alexander Kristoff and Nacer Bouhanni."
Post update
Right then, that's stage four done and dusted. the race report is taking shape so it's time to move on to stage five...
Martin in yellow
Yellow Jersey
Classification
1. Tony Martin (Ger) Etixx - Quick-Step 12hrs 40mins 26secs
2. Chris Froome (GB) Team Sky +12secs
3. Tejay van Garderen (US) BMC Racing +25secs
4. Tony Gallopin (Fra) Lotto +38secs
5. Peter Sagan (Svk) Tinkoff - Saxo +39secs
6. Greg Van Avermaet (Bel) BMC Racing +40secs
7. Rigoberto Uran (Col) Etixx - Quick-Step +46secs
8. Alberto Contador (Spa) Tinkoff - Saxo +48secs
9. Geraint Thomas (GB) Team Sky +1min 15secs
10. Zdenek Stybar (Cze) Etixx - Quick-Step +1min 16secs
Stage 4 result
1. Tony Martin (Ger) Etixx - Quick-Step 5hrs 28mins 58secs
2. John Degenkolb (Ger) Giant +3secs
3. Peter Sagan (Svk) Tinkoff - Saxo
4. Greg Van Avermaet (Bel) BMC Racing
5. Edvald Boasson Hagen (Nor) Team MTN
6. Nacer Bouhanni (Fra) Cofidis
7. Jacopo Guarnieri (Ita) Katusha
8. Tony Gallopin (Fra) Lotto
9. Zdenek Stybar (Cze) Etixx - Quick-Step
10. Bryan Coquard (Fra) Europcar
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Jess Parsons: CRACKING end to today's stage on #TDF2015 TT specialist @tonymartin85 takes it! Moments like that are what cycling is all about.
Stuart Mitchell: Sky won't care, they wanted to get through the day, and it's only 12 seconds. Froome will reclaim yellow in a few days.
Andrew Harvey: I'm sweating just sitting at my desk reading the updates! Fantastic effort from @TeamSky and stage winner @tonymartin85! #bbccycling
Post update
Rob Hayles
Former GB cyclist on BBC Radio 5 live sports extra
"Today shows why Dave Brailsford set the team set up around Chris Froome with a Classics orientated squad. It was particularly for this stage. This was the key one in the opening week where they could have lost significant time but they've come up trumps.
"I'm glad there were no big losses today though because it's kept everyone in the chase, the top four in particular. It's shaping up to be an incredible race."
Post update
I must admit to missing that bit. I saw Martin suffer a problem with around 20km remaining. It appears as though he got on Matteo Trentin's bike and rode to the finish on that.
And for those of you wondering why Team Sky and Chris Froome will be happy to be rid of the yellow jersey - it's mainly to do with the pressure of holding it. They will now be able to relax over the next few days and worry about winning it back when they hit the Pyrenees next week.
Post update
Here's Tony Martin's thoughts on that finish: "After having a flat tyre and changing bikes I was thinking about the next stage. But I got back in and with 5km remaining everybody was looking round and not pulling so I went for it. A thousand thanks for my team. I am super happy."
Post update
Big hugs and kisses all round in the Etixx - Quick-Step team. It's been an eventful few days for Mark Cavendish and his team-mates. After messing up the sprint on stage two, they got it so, so right today.
The top four
Martin's German team-mate John Degenkolb was second over the line, one second adrift, with Peter Sagan third and Greg van Avermaert fourth.
Post update
Rob Hayles
Former GB cyclist on BBC Radio 5 live sports extra
"What an incredible finale for Tony Martin. He has just missed out on the first three stages. He deserves it so much. He started out just one second behind Chris Froome and Team Sky will not be bothered to lose the yellow jersey."
Martin wins and takes yellow
Yellow Jersey
Classification
Tony Martin wins stage four and takes the yellow jersey.
Post update
Martin is into the final 200m, he's going to win this and ride into the yellow jersey...
1km remaining
Mark Cavendish is hovering in the train, about eighth wheel. He does not need to worry about a leadout because his team-mate Martin is going for a solo victory. But Martin is only five seconds clear as he enters the final kilometre.
2km remaining
Tony Martin is a three-time time trial world champion so he is used to putting in a big effort and holding on. He has 2km to go. If he wins, he is in the yellow jersey.
Post update
Rob Hayles
Former GB cyclist on BBC Radio 5 live sports extra
"I don't think Team Sky will be chasing. They won't mind if they lose the yellow jersey today."
Martin attacks
There are two right-angled bends in the final kilometre and the last 600m is uphill. Doesn't sound perfect for Cav, who prefers the flatter sprint.
And while everyone is looking at each other, Tony Martin goes on the attack.
Post update
Rob Hayles
Former GB cyclist on BBC Radio 5 live sports extra
"Mark Cavendish is in this lead group. On this finish if he's got the legs, he could be getting his hands in the air. But his Etixx - Quick-Step team have got Tony Martin to think about as well."
5km remaining
We are 5km from the finish and all the big favourites are in the mix. As exciting as the cobbles have been, they've not been quite as divisive as last year.
I don't think any of the big four will be looking for a stage win. So it's down to the likes of Sagan, Degenkolb, Van Avermaert etc...who's your money on?
Geraint Thomas and Chris Froome are back on the front of the bunch. I want what G has for breakfast.
Group back together
Incredible scenes on the road. Froome, Nibali and Valverde are in this leading group. Peter Sagan has been distanced. Can they keep the speedy Slovakian adrift?
Geraint Thomas, refreshed from taking a quick break, takes up the pace again but the group behind is catching them.
10km remaining
There is a group of around 10 riders clear of the bigger group. Geraint Thomas finally sits up for a breather, leaving Froome out front. Froome flicks an elbow, indicating he wants someone else to take over.
Attacks galore
Nibali goes first but he is passed by Zdenek Stybar. Froome goes into Nibali's wheel and then goes past the Italian.
Geraint Thomas chases down Stybar and then looks over his shoulder and waits for Froome and the two of them emerge from the cobbles right at the front. Terrific riding by G.
Post update
Will anyone make a dash for it on this final section of pave? Astana again take up the pace, but once more Geraint Thomas is up at fourth wheel and Chris Froome slips in behind him. There are about 40 riders strung out behind. Vincenzo Nibali sidles up alongside Froome. Alberto Contador and Nairo Quintana are still in the leading group.
Post update
Rob Hayles
Former GB cyclist on BBC Radio 5 live sports extra
"Team Sky have played a blinder so far."
15km remaining
The riders all look like they've been down a coal mine as they enter the final 15km to the finish. Just one cobble section to come. It's 2.3km long and there will be 7km remaining when they get back on the asphalt.
Post update
Matt Slater
BBC Sport at the Tour de France
"As expected, it has all come to life on the cobbled run-in to Cambrai. Chris Froome's rivals have all had a go at trying to put him under pressure but the Team Sky rider's wobbliest moment so far has been when he rode into the gutter on a relatively decent stretch of road.
"It is the French favourites who have been having the hardest time of it, which is ironic given the fact that we are actually in France now.
"Pierre Rolland was dropped by the leaders' pace relatively early but Thibaut Pinot has really been in the wars and was last seen slamming his bike onto the road in frustration. He badly needs a new steed."
Martin mechanical
Meanwhile, the leaders are off the cobbles. Just one section of Pave remaining. Simon Brotherton reports that Tony Martin is out the back. Unclear whether a puncture or a mechanical for the German. He is just one second behind Chris Froome in the overall race standings remember.
Pinot is off his bike again and he slams it into the side of the road. Frustrations boiling over for the Frenchman. He is three minutes behind the leaders now.
Post update
Rob Hayles
Former GB cyclist on BBC Radio 5 live sports extra
"I wouldn't imagine it will be bon appetit for the FDJ team when Pinot turns up for tea tonight."
Pinot punctures
The choking dust will not be helping the riders. At the back of the leading bunch, France's big hope for this year's Tour, Thibaut Pinot, comes to a halt. Looks like a puncture at the worst possible time. Pinot is looking distraught as he gets a shove from his mechanic.
Post update
There are about 30 riders in a long line cycling right up the middle of the road. Tinkoff-Saxo put a man on the front. Geraint Thomas is around 12th with Chris Froome still hugging his wheel as they head though the fields of northern France.
Post update
It looks like Geraint Thomas is last-Skyman standing with Chris Froome as they get back on to the tarmac. Thankfully no rain and no fallers so far but this is punishing racing as the riders swing right on the penultimate section of cobbles.
25km remaining
Here we go on the cobbles again. Vincenzo Nibali attacks again but almost rides into the back of Tony Gallopin. The Italian gives the Frenchman a shove in the backside.
John Degenkolb takes up the pace with Geraint Thomas hanging on in there, with Chris Froome following his every move.
Froome almost falls
Having just typed that, Froome comes within a whisker of being shovelled off his bike. That was close. It was a Katusha rider that nudged him wide and he clipped the kerb before being bounced back into the peloton.
Froome in position A
Geraint Thomas and Chris Froome momentarily drop back into the middle of the leading bunch but they drift out to the right and into open road and from a precarious position, they are in position A again.
Pave sector three coming up. It's 1500m long and followed quite quickly by pave two which is 3.7km.
30km remaining
Lotto Jumbo, who have classics specialist Sep Vanmarcke in their midst, storm to the front of the peloton but Nibali is having none of that and puts his Astana boys back up there.
Post update
BMC Racing have several riders in the front group, riding for Greg van Avermaert. Geraint Thomas is continuing to show Chris Froome the way. There are a lot of nervous glances around. The second, much larger, bunch on the road is around 35 seconds adrift.
Kristoff punctures
The riders are strung out again. Bad news for Alexander Kristoff who has suffered a puncture. That could end his hopes of a stage win. Andre Greipel, the leader of the points competition, is also losing ground.
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Small Man Peaky: This is insanity. Froome is going to need Stannard and G to get him to the safety. I hope Martin gets the yellow. #bbccycling
35km remaining
BMC Racing's Greg van Avermaert is looking interested, as is Movistar's Alejandro Valverde. There are about 40-50 riders left in the front group as they career on to pave sector four.
Post update
They are off the cobbles and this is like the finish to a one-day classic. No sizeable gaps opening up. Zdenek Stybar of Etixx - Quick-Step takes up the pace and he is joined by team-mate Matteo Trentin, while Mark Cavendish is also up there. Will they be successful in getting Tony Martin into yellow?
I can see the white sunglasses of Geraint Thomas in about fourth wheel.
Nibali attacks
The breakaway is swallowed up and Vincenzo Nibali immediately goes on the attack with his Astana team-mates. Ian Stannard and Geraint Thomas respond. And Chris Froome muscles his way into sixth place in the line.
More cobbles
Team Sky's Ian Stannard swings across in front of an Astana rider to take on the peloton pace-setting. Chris Froome, like he is on a leash, is inches behind. Geraint Thomas also up at the front. The Welshman loves the cobbles and is the perfect man to chaperone Froome as they enter sector five of the pave.
Post update
All of the big boys appear to be together though. We are 5km from the next section, which is 1.6km long.
Nervy times on the cobbles
Team Sky are alert to the danger and close it down and Nibali sits up. Chris Froome is in the wheel of Ian Stannard and Geraint Thomas is close by. Nervy times for the big general classification riders.
Post update
Team Sky have got helpers every 500m along the sections of pave.. Etixx - Quick-Step are leading the way. Tony Martin is the man on the front. Vincenzo Nibali is behind him and as they come off the cobbles, his Astana team-mate gives him a nudge and off they go.
The break almost done
It's reached that time of day where the breakaway is going to be swallowed up, so it's time for a final namecheck as they enter the second of today's seven cobbled sections.
The men in the break are: Lieuwe Westra (Astana), Thomas De Gendt (Lotto-Soudal) who initiated the breakaway, Perrig Quemeneur (Europcar) and Frederic Brun (Bretagne-Seche Environnement).
There are clouds of dust being whipped up by the motorbikes and cars.
Rain starting in Cambrai
BBC Radio 5 live sports extra
BBC Sport's Gemma Sterba is with Rob and Si in the commentary booth. She's just emailed in to tell me: "Just been on the coffee (and cookie run) for the boys. Few rain drops... couple of brollies going up. Wish I hadn't left my jacket in the car now."
Dowsett down
There has been a crash or two in the peloton. Nothing major. A slightly damp road and a tight right-hander to blame. It was a low-pace crash and there are no casualties.
At the back Britain's Movistar rider Alex Dowsett is down and he has a bloodied elbow. He will get back on though. The leaders are just one minute clear as Vincenzo Nibali's Astana team up the pace.
50km remaining
The list of potential winners today is long. Peter Sagan, John Degenkolb, Greg van Avarmaert, Tony Gallopin, Tony Martin.
And in the bigger picture, will Vincenzo Nibali be able to get any time back on race leader Chris Froome? How will Alberto Contador and Nairo Quintana fare? We are about to find out folks.
55km remaining
Serious race heads on in the peloton. Andre Greipel, in his bright green points leader jersey decides the pace is not high enough, and he stomps on his pedals. He is looking to repay the help he has been given so far in the race - maybe Tony Gallopin is the man he is working for.
Here comes the rain
Simon Brotherton
Cycling commentator on BBC Radio 5 live sports extra
"One or two spits and spots of rain landing on the window."
The cobbles are coming
The pace is being lifted in the peloton and all the teams are organising their trains. Team Sky are hugging the left-hand side of the road. Alberto Contador's Tinkoff-Saxo boys are down the middle with Nairo Quintana and Alejandro Valverde's Movistar.Kristoff's Katusha down the right. Just 10km to the cobbles.
60km remaining
Rob Hayles reminds us that the general calculation for catching a break is one minute for every 10km. So the four escapees, who are three minutes clear, have little chance of making it to the finish. Although, if the cobbles get damp...
Post update
Now, from a neutral point of view, a bit of wet would spice things up nicely for the finish. I'm not suggesting I want anybody to come off and get injured but a couple of slips and slides would liven up the final 50km.
I can guarantee none of the riders will want it. The sky is slate grey though...
Post update
Simon Brotherton
Cycling commentator on BBC Radio 5 live sports extra
"Looking up to the skies, I'd say it's 50-50 as to whether it rains at the finish before the riders get here."
Rob Hayles adds: "It's a big game changer if it does rain. It makes the cobbles extremely slippery."
In the break
The leading quartet are continuing along their merry way, tapping out a decent pace. Their lead is still around three minutes as they approach 65km to go. Just 20km to the next section of pave.
Post update
Rob Hayles
Former GB cyclist on BBC Radio 5 live sports extra
Rob found a bit of coverage from the big crash on yesterday's stage, seen through the eyes of a Lotto NL Jumbo mechanic. It's an interesting insight into life at the Tour.
Post update
BBC Radio 5 live sports extra
Jess Parsons (14:43) will no doubt be pleased to hear that 'Big Si' is up and running on BBC Radio 5 live sports extra. Mr Brotherton is joined in the commentary booth by Rob Hayles. Click on the link above to listen in.
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Ieuan Llyr Jones: (re Elliot 14:27): Big Mig did. 1992. Won the prologue, lost next day. Won it back 11 days later. #bbccycling
Mark Williams: In addition to Nibali last year, also Lance Armstrong in 1999, 2002, 2004 & 2005. Not that they count any more!
Big Mig is five-time Tour winner Miguel Indurain, the only rider (since Lance Armstrong's seven on the trot were deleted from the record books) to win five successive races. The Spaniard was dominant from 1991-95.
Why it's good to be at the front
The race is calm again. But we are only 30km from the cobbles. And that means the action will start in around 20km as the peloton picks up the pace to put their riders in position.
Geraint Thomas told BBC Sport before the race that being up near the front is good for several reasons. Firstly, the roads narrow so there's less space. Secondly, you are less likely to be involved in, or caught behind and therefore delayed by a crash.
The other bonus for Team Sky is that because they have Chris Froome in yellow, their team car is first in the line of 22 that follow the race. Therefore, if Froome crashes, or punctures, his car will be with him the quickest.
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Matt Lewis: (re Elliott 14:27) Vincenzo Nibali, last year? Got it stage 2, lost it stage 9, regained it on stage 10 and kept it till the end.
Jess Parsons: Peter, loving the live text but there's nothing like Big Si getting me animated all afternoon whilst I sit at my desk! #bbccycling
Glad to be the warm-up act Jess. Not long to wait for 'Big Si' and Rob Hayles. Live coverage on BBC Radio 5 live sport extra from 15:00 BST. Buttons to push at the top of this page.
Sagan suggests an attack
A nice touch from Peter Sagan there. He follows Cavendish, Greipel and Degenkolb over the line and while they have been battling for points, the peloton has been left a couple of hundred metres behind. Sagan, with a smile, gesticulates that they should go off on the attack.
His cheeky request is met with stony looks. He shrugs, laughs and they all drop back to the comfort of the bunch.
Cav wins minor sprint place
Green Jersey
Classification
And it is Cavendish that takes the 11 points. Behind him Greipel and John Degenkolb have a little coming together as they battle to get on the wheel of Cav. Six of one and half a dozen of the other but it knocks Greipel off his stride and he will be seventh over the line.
Post update
Green Jersey
Classification
Thomas de Gendt of Lotto-Soudal suddenly takes off and he takes the 20 points (and 1500 euros prize money). De Gendt is a team-mate of Andre Greipel and he is two minutes behind and in the middle of his sprint. Mark Cavenidish is in the shadow of his lead-out man Mark Renshaw....
Post update
Green Jersey
Classification
Up the road the four in the break are not looking too interested as they edge into the outskirts of Havay. Astana's Lieuwe Westra is doing the least work, he has bigger battles to come when his team leader Vincenzo Nibali decides to make use of him.
Post update
Green Jersey
Classification
The intermediate sprint in Havay marks this year's entry into France. The sprinters are lining themselves up in the peloton...
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Elliott Riddle: Has any rider ever secured yellow this early on, subsequently lost it, and then regained it again come the Champs-Elysees? #bbccycling
Elliott, I'm sure one of your fellow readers will supply you with an answer...
Sprint coming up
Green Jersey
Classification
It's all rather quiet out on the road and that, given the carnage of yesterday and the chaos to come, is probably no bad thing.
The leaders are approaching the intermediate sprint in Havay. The four in the break will get 20, 17, 15 and 13 points. The first man from the peloton will get 11. Expect Mark Cavendish, Andre Greipel, who leads the classification, and Peter Sagan to be interested.
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Tom Oxley: Cobbles is doing the pave a disservice, these things are the size of a baby's head.
Ken Smith: Hoping Tony Martin can pinch some bonus seconds & therefore yellow. Sky don't want it now! G is in top form, go G (that's Geraint Thomas).
Amber Gahan: It's that time of year again when I am glued to #bbccycling text commentary of Le Tour at work
In the break
Lieuwe Westra, the Astana team-mate of Vincenzo Nibali, drops back to his team car and hops off his bike. He too is changing for a more cobbles-friendly racer.
He is of course in the break and the quartet's lead is hovering around the the three minute mark with 100km, or about 60 miles if your prefer, of today's stage remaining.
Panic attacks for Matthews
One rider not looking forward to the cobbles is Michael Matthews. The Orica GreenEdge rider went down and took a couple of layers of skin off, prompting Rob Hayles to say on commentary: "Matthews looks like he may well have lost some of his tattoo on his back. That will take some touching up in the coming months."
Matthews says: "My goal is just to get to the cobbles and see how it goes and make the time cut basically. Last night I was really struggling to breathe. That wasn't great. I had little panic attacks I could not breathe, it was quite scary."
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Andrew Harvey: Favourite's for this stage have to be Kristoff and Degenkolb. Both had quality classics campaigns and should finish strongly. If Stannard and Thomas can get Froome safely to the end then he should stay in Yellow. Whether he wants to is another matter. #bbccycling
In the break
The pace has been knocked off by the chasing peloton after that burst of speed into the cobbles. That means our four plucky riders out front are extending their advantage again and are now more than three minutes clear.
They are: Lieuwe Westra (Astana), Thomas De Gendt (Lotto-Soudal) who initiated the breakaway, Perrig Quemeneur (Europcar) and Frederic Brun (Bretagne-Seche Environnement).
Bike changes
A couple of riders have changed their bikes after that opening section of cobbles, also known as pave. Alberto Contador was one of them.
It was a planned move and he will have slightly wider tyres and better suspension to absorb the bouncing around on the cobbles. Team Sky are also on specialist bikes for today's stage. It's all about marginal gains.
The riders have 110km remaining - the next section of pave comes 46km from the finish.
Crestfallen Cancellara
Fabian Cancellara has taken to Twitter as he contemplates the end of his 2015 Tour with a fractured back. The rider who goes by the nickname Spartacus, wrote: "One day you win one day you lose but I will not give up on what happen #spartacus @TrekFactory thanks for all your messages and comments.
"On the way home with lot's of pain all over my body. Doesn't feel nice to crash at 75km/h. This morning it felt horrible to stand out from bed."
Post update
Here's a slightly closer look at the finish that awaits our riders in about two and-a-half hours..
Post update
BBC Radio 5 live sports extra
The BBC Radio 5 live team have stolen a march on the peloton. They're at the finish line in Cambrai. Guess what - there's cobbles.
You can listen to commentary from 15:00 BST on BBC Radio 5 live sports extra and on this page.
Post update
Mark Cavendish is on the front of the peloton. He will be working for his Etixx - Quick-Step team-mate Tony Martin, who is second behind Chris Froome - one second adrift.
The Manx Missile is burying himself here to help his team-mate. Cavendish was clearly gutted to miss out on winning stage two but that is a long way behind him now.
The first section of pave is safely negotiated. An army of team mechanics wait at the end with tyres held high just in case any of their riders have punctured. But all seem safe and on we go.
Cobbles section one
Here we go on to the cobbles. They look pretty smooth to me, in comparison to some of the sections in Paris-Roubaix. The leaders enter the first of seven sections 70- seconds ahead of the peloton.
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So, who are you backing for the stage win today? Who will cope best with the cobbles? Will Froome hold on to the yellow jersey?
BBC Radio 5 live sports extra commentator Simon Brotherton has gone for: John Degonkolb to win, Greg van Avermaert to take the yellow jersey.
Matt Slater has gone for the same as Simon.
Rob Hayles is backing Tony Gallopin for the stage win and can't make up his mind between Van Avermaert and Peter Sagan for the yellow jersey.
You can hear live commentary of the closing stages from 15:00 BST by clicking the relevant link at the top of this page.
Text me your thoughts on 81111. Or tweet #bbccycling
Post update
Matt Slater
BBC Sport at the Tour de France
"Some day soon we are going to get a nice, routine, first-week-of-the-Tour stage and we can all take a long lunch and change for dinner. It won't be today, though.
"The longest stage, seven sections of cobbles, wind, a forecast of rain and the certainty of sustained aggression.
"With Froome in yellow, earlier than expected, his rivals must stop Team Sky from gathering any more momentum.
"Nairo Quintana simply has to stop losing chunks of time, Alberto Contador could use a relative win and Vincenzo Nibali needs a repeat of last year's virtuoso demonstration of strength and skill.
"The Italian already has team-mate Lieuwe Westra up the road. The foundations for an attack have been laid."
In the break
Four riders broke clear from the peloton in the early knockings of today's stage and were allowed to build up a lead of approaching 10 minutes but they have been pulled back in to a more manageable two minutes.
The men in the break are: Lieuwe Westra (Astana), Thomas De Gendt (Lotto-Soudal) who initiated the breakaway, Perrig Quemeneur (Europcar) and Frederic Brun (Bretagne-Seche Environnement).
Westra is the interesting one because he is a team-mate of one of Froome's main rivals Vincenzo Nibali - and the Italian is decent over the cobbles.
Here come the cobbles
The riders are already off and pedalling today and have completed around 100km of the 223km route. They are approaching the first of the seven cobbled sections and the pace is high.
True to Geraint Thomas's prediction, it's all about being first into the cobbles and the Team Sky train is right on the front of the peloton. They have around 5km to the first section. The remaining six all come in the final 50km
Geraint's guide to stage four
"Last year Team Sky didn't have the best of days on the cobbles, when Froome was forced to abandon, but personally I did pretty well. It's definitely a stressful day, but I enjoyed it and I'm looking forward to it again.
"If it's dry it won't be as bad, but if it rains like last year there could be big time gaps again. It's certainly a big day for the teams with riders competing for the overall victory.
"Everybody will be stressing about that and you've just got to concentrate on being in a good position at the front. The only problem is that's what 180 other riders want to do. It's all about fighting for position for when most of the cobbled sections start in the final 50km or so."
Cobble time
It is the longest stage of this year's race. A 223.5km (138.8 miles) ride from Seraing to Cambrai as the race leaves Belgium and crosses into France for the first time.
There are no significant climbs to negotiate today but what we do have is cobbles. Almost 13.5km spread across seven sections. If you've watched Paris-Roubaix and the one-day Spring Classics you'll be well aware of the chaos that the cobbles can cause.
If you're new to all this, you are in for a real treat today. Check out what Britain's Team Sky rider Geraint Thomas told us in his Tour de France stage-by-stage guide...
Post update
Marginal gains has been one of British Cycling saviour Sir Dave Brailsford's mantras over the years and it certainly paid dividends on stage three. But enough of stage three. That was so yesterday.
Today is all about stage four...
Marginal gains
Maybe Froome should have held back by 0.07 seconds. If he had been awarded a time one second behind stage three winner Joaquim Rodriguez, instead of the same time, Germany's Tony Martin would be in the yellow jersey today.
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Rob Hayles
Former GB cyclist on BBC Radio 5 live sports extra
"Team Sky rode extremely well. Chris Froome, when he went with around 500m to go was very early but he set himself up to finish ahead of his main rivals and Alberto Contador was the big loser (finishing 18 seconds adrift of Froome).
"It's very early in the race for Froome and his team-mates to defend the yellow jersey. They have probably got themselves in this position without wanting it."
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BBC Radio 5 live sports extra
Also on the podcast are our new race leader Chris Froome and his Team Sky boss Dave Brailsford.
Froome admits he is "surprised" to be in yellow and that "it's really early days".
Brailsford said getting the race lead was reward for "fantastic effort".
He added: "It's a big deal getting the yellow jersey, the most famous jersey in cycling and every time you get the opportunity to wear it you should respect it and be super proud of having it in the team and I know that's what we will be."
Orica hardest hit
BBC Radio 5 live sports extra
Matt White, sporting director at Orica GreenEdge, told BBC Radio 5 live Sport Extra that it was "a fair decision" to stop the race. Several of his riders were caught up in the crash and he lost Simon Gerrans and Daryl Impey from his nine-man squad.
Britain's Simon Yates, who also rides for the Australian-based outfit, went down in the crash but he recovered to finish eighth on the stage.
"Rider safety comes first," said White. "These rules have to be taken on merit from crash-to-crash."
You can listen to more from Matt on the podcast.
Prudhomme defends stopping race
The race was stopped for a little over 10 minutes to allow race doctors to attend to the injured, which caused a little confusion among those still riding.
Team Sky's Geraint Thomas said that he and a lot of the riders were not totally sure why the race had been stopped until they reached the finish.
But most people were in agreement that race director Christian Prudhomme had made the right decision in calling a halt to proceedings.
Prudhomme defended his actions, saying that although the main crash was "more spectacular than serious", the four ambulances and two doctors cars that follow the race were needed to treat the injured.
He added that he had no option but to stop the stage because "you must have medical assistance vehicles behind the race".
Cancellara & Impey abandon
Switzerland's Fabian Cancellara was the most high-profile casualty. It turns out the race leader, who was catapulted head first into a ditch, finished the stage despite fracturing two vertebrae in his back.
South Africa's Darly Impey was another to call it a day - although he rode the final 60km or so with a dislocated collarbone.
Australia's Simon Gerrans, Dutchman Tom Dumoulin, Dmitry Kozontchuk of Russia and France's William Bonnet, who sparked the chaos by being the first to come off his bike, are also out.
Andreas Schillinger of the Bora-Argon 18 team was also a non-starter this morning.
Tour carnage
Hello and welcome to live text and radio commentary (from 15:00 BST) of stage four of the 2015 Tour de France. It was another crazy Tour day on Monday with crashes, broken bones, dislocations, retirements and a tremendous finish up the impossibly steep Mur de Huy.
I'll move on to stage four after a brief look back at the carnage of day three which resulted in seven abandonments.
Froome in yellow
Yellow Jersey
Classification
"I would much rather be in this position than having to make up time on my rivals," said British rider Chris Froome after surprising himself by taking the overall lead of the Tour de France on Monday's hectic third stage.