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Sheffield United are bottom of the Premier League; Plymouth are 11th in League One
Four-time winners Sheffield United last won the competition in 1925
Sheffield United beat Bristol Rovers in round three; Plymouth overcame Huddersfield
Swansea City 3-1 Nottingham Forest
Liam Cullen slips as a blocked shot almost falls for him in the box. He manages to poke it out to a team-mate and Ryan Manning then has a strike pushed away by the keeper.
Sheffield United 2-0 Plymouth Argyle
Michael Brown
Former Sheffield United midfielder on BBC Radio 5 Live
At some point Argyle have got to take a few risks. It's far too comfortable for United at the moment.
Sheffield United 2-0 Plymouth
Billy Sharp gets clear of the Plymouth defence, cuts inside - but blazes over.
Here's the chance he did put away at the start of the second half.
Swansea City 3-1 Nottingham Forest
Joe Worrall actually looked to pull out of the way of Ryan Manning as he seemed to try and catch his leg.
Very soft. I'm not having it.
Matt Grimes (Pen - 61 mins)
Matt Grimes gets his second of the day, slotting it coolly into the left corner from the spot as Jordan Smith dives the other way.
Swansea City 2-1 Nottingham Forest
Oh.
Joe Worrall is penalised for a foul on Ryan Manning just inside the box. Didn't look like much but Kevin Friend is insistent.
Brighton 2-1 Blackpool
Brighton look like they want to avoid any more upsets as they bring on first-team regulars Neal Maupay, Leandro Trossard and Solly March for Alexis Mac Allister, Steven Alzate and Percy Tau.
West Ham 3-0 Doncaster
They've probably given themselves way too much to do here, but Doncaster are at least giving it a go. The ball is swung into the box and falls for Matthew Smith, who fires a rising shot against the bar and over.
Swansea City 2-1 Nottingham Forest
Such quick feet from Joe Lolley in the box to set-up Anthony Knockaert for that Forest goal.
He was fouled too so Forest would have had a penalty had Knockaert not ripped the net apart!
Anthony Knockaert (56 mins)
Forest are rewarded for their bright start to the second half.
They press high up and force Swansea into a mistake. A combination of Lyle Taylor and Joe Lolley force it before teeing up Anthony Knockaert in the middle.
He thumps it into the net first time from close range. He punches the air and urges his team on.
Steven Alzate (58 mins)
Oh big deflection!
Harsh, very harsh on Blackpool as Alexis Mac Allister's shot from distance takes a massive deflection off Steven Alzate's back to fly into the opposite side of the net to goalkeeper Chris Maxwell.
Blackpool are going to have to come from behind again.
Nahki Wells (58 mins)
Millwall had started the second half well but they're right up against it now. Nahki Wells has a free-kick about 25 yards out, hits it over the wall and it takes a huge deflection and bounces in.
That may end up going down as an OG.
#bbcfootball or text 81111 (UK only)
Seb Chambers: Sorry but that was a pen in my opinion for Sheffield United. His arms have gone in a circular motion which looks like an illegal attempt to block the ball.
David Ellis: Just watching #SHUPLY, a penalty was referred to VAR and you heard the VAR ref talking to the on field ref about the decision like they do in rugby. Is this a new feature? I think I like it!
Brighton 1-1 Blackpool
Set piece worries again for Blackpool as Adam Webster meets another Brighton corner but his header deflects off a Blackpool player and behind.
Swansea City 2-0 Nottingham Forest
Connor Roberts cuts in from the right and tries to curl it into the far corner. Straight into the keeper's arms.
He knows he should have done better.
Swansea City 2-0 Nottingham Forest
It's brilliant defending from Ryan Manning as he prevents Loic Mbe Soh getting a shot away in the box before putting pressure on Anthony Knockaert.
Eventually, it goes out for a Forest corner but Joe Worrall's header at the back post is again deflected behind.
Better start to the second half from Forest.
Callum Styles
This game was never going to end 0-0. Barnsley have knocked the loudest and it is they who lead thanks to a fine finish from Callum Styles, who had to gather the ball, turn and fire low past Krul.
Andy Butler own goal
Nightmare for Butler!
The corner is swung to the near post and nodded on. Butler is on his own line and needs to clear but gets it all wrong and slices the ball into his own net.
That is game, set and match.
West Ham 2-0 Doncaster
Balcombe is staging a one-man intervention to prevent West Ham running away with this. Noble hooks an effort on target from range and the Rovers keeper is forced to tip it over.
The corner is taken short and then lobbed to the back post, where Soucek steers a header that is... guess what? Yep, saved by Balcombe.
Brighton 1-1 Blackpool
Brighton are almost caught out by a similar move that led to Blackpool's equaliser but this time Gary Madine is just unable to connect with the cross from the left.