It is a yellow onslaught. Arsenal so on top here, it can only be a matter of time before they grab a second. How ruthless are the Gunners feeling?

West Ham manager Sam Allardyce, speaking to BBC Sport: "I knew it was going to be a tough game and it was exactly that. Bristol City showed why they are where they are in the league with their form today. We found it difficult to get our passing routine going but when the one chance popped up it was all we needed to get a very important victory. Bearing in mind what has happened this weekend, it is a big result for us."

We've not seen much of this this weekend, the Premier League side totally in control. Aaron Ramsey dives at goal, his powerful shot deflected just past the post. From the corner Olivier Giroud is close to bundling in at the front stick.

That's better from Villa! Christian Benteke cuts in from the left, could shoot but goes on, could shoot but goes on, could shoot but goes on - and then finds Carles Gil who whips a shot into the side-netting.

Much better.

Any Brighton gameplans of containment or frustration are out of the window instantly. There is a shell-shocked air in the stands.

Former Tottenham manager David Pleat
BBC Radio 5 live at Amex Stadium

"Theo Walcott was first to it and he hit it across the goalkeeper for a lovely finish. Albion now really have a mountain to climb. The goal is quite a body blow but they must maintain their composure and discipline."

Back under way at Villa Park. Can Villa score?

Well that didn't take long! Arsenal lead before Brighton had hardly had a touch. Calum Chambers gets clear on the right and puts a cross in to the near post, Theo Walcott's touch is perfect to bring it under control and he cracks the shot into the far corner from 12 yards. Wonderfully taken.

Image source, Getty Images

Game on at Brighton.

West Ham defender Winston Reid, speaking to BBC Sport: "In the first half we were under pressure. It is always difficult coming to these places, Bristol City put in a good performance but we put in a clean sheet and Diafro Sakho got the goal, so it was good. We wanted to come here and finish off the game and we did, even though we had to wait for that. The manager spoke before about the upsets yesterday, and we did not want to be one of them."

Fabian Delph, with the ink still damp on his new contract, is out applauding the fans at Villa Park. How important may keeping him prove to be?

And the winner is...

Former England midfielder Danny Murphy
Match of the Day at Ashton Gate

Image source, Reuters

"Diafra Sakho is the match-winner and he has been full of goals this season. 10 goals in 16 games I think it is. As soon as he came on he showed a brilliant bit of skill and was unlucky not to score when his shot hit the bar. For Bristol City, Matt Smith was a pest all afternoon. He had one clear chance when he really should have done better."

We will continue to bring you the best reaction and analysis of West Ham's victory as we tick over to Arsenal's trip to Brighton. A record crowd at the Amex? Arsenal are in the tunnel awaiting their opponents. Yellow kits today for the holders.

Brighton manager Chris Hughton on BT Sport: "The shocks highlight to everybody the surprises in the cup and what it means to everybody. We are a team in decent form.

"There is great potential at this club. The stadium we have here - it's a record crowd for the stadium - and the training facilities are fantastic. We are not in a great position in the league though."

Arsenal manager Arsene Wenger on BT Sport: "The fourth-round shocks were a huge surprise. That's English football - the academies of the clubs in the lower divisions have the players prepared physically. My team selection today is about some players having played many games rested and players who haven't played many games that are playing. Ozil and Walcott are physically ready to play, but they can only show that in the game."

Former West Ham striker John Hartson
BBC Radio 5 live at Ashton Gate

"West Ham will be delighted. They were never going to come here and get an easy game. Bristol City are flying high in League One, they can now concentrate on promotion. They are a young team, and they can take plenty of positives from today's result. West Ham, meanwhile, will think they can go all the way in this competition."

Seventh-placed West Ham are the highest-placed Premier League team definitely through to the fifth round of the FA Cup.

Could they win it?

Manchester United have a replay, and Arsenal are about to play...

West Ham just about do enough. They are into round five thanks to that late header from Diafra Sakho.

Former England manager Graham Taylor
BBC Radio 5 live at Villa Park

"The home fans are looking at a Premier League side playing a Championship side and they are even, so you are going to get a few boos, that's natural. This is not Bournemouth's first team but they have played well, they move the ball very quickly and they move it forward when they can. But Villa have had their moments too. A draw is fair at the moment."