A practice start for Lewis Hamilton - Mercedes have had a couple of those in Belgium. He'll be keen to get away well this weekend after some poor starts for the championship leader in recent races, especially with the new rules.

Any improvement from Williams? Nope. Valtteri Bottas, fastest in FP3 last year in Belgium, fails to produce a particularly quick lap and a 1:50.179 is only good enough for ninth.

No soft tyre run for Fernando Alonso, he is not getting out at all in this session because of that power unit problem, but Jenson Button has the yellow-walled tyre on and goes 15th fastest on his first run.

Sebastian Vettel is out on his qualifying sim run and goes third fastest, six tenths down on Lewis Hamilton's benchmark. Vettel's team-mate Kimi Raikkonen is fourth, with Sergio Perez fifth.

Someone's happy with that flex of the muscles by Mercedes.

...it is Lewis Hamilton who goes fastest, a 1:48.984 and he is almost half a second quicker than his team-mate.

Nico Rosberg pulls it out of the bag in the final sector to go four tenths faster than Daniel Ricciardo, a 1:49.482 popping him into P1.

Meanwhile, Lewis Hamilton is also on a lap on the softs, who will be fastest of the two Mercedes...?

Nico Rosberg rolls out with the softs bolted on and begins his qualifying simulation run.

Just three drivers out on track - Daniel Ricciardo, Daniil Kvyat and Max Verstappen on used softs - but there is movement at Mercedes. Expect it to get very busy soon enough as we enter the final quarter of an hour.

Jennie Gow
BBC pit-lane reporter

"I've just been speaking to the guys at Pirelli - not about exploding tyres, but predictions for the race, they reckon it will be a two-stop race. The soft tyre is looking to be about 1.7s quicker."

Not too many teams have actually decided to make the switch to softs, with just the Red Bull pair of Daniel Ricciardo and Daniil Kvyat and the Toro Rosso of Max Verstappen on that compound with 20 minutes remaining.

The track is getting busier with 13 cars out there, Jenson Button's McLaren one of them. He goes 17th fastest with his first timed effort, a 1:53.297.