Postpublished at 83 mins
Southampton 1-0 Arsenal
James Ward-Prowse's delivery is surprisingly disappointing as it hits Nicolas Pepe painfully on the face.
It does fall for Danny Ings, who takes a snap shot at it, but it's straight at Bernd Leno.
Use play icon to watch live coverage alongside commentary from Martin Fisher & Rachel Brown-Finnis
Also listen to commentary from BBC Radio 5 Live, BBC Radio Sheffield & BBC Radio Devon
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
Emma Sanders, Phil Dawkes, Gary Rose, Tom Rostance and Shamoon Hafez
Southampton 1-0 Arsenal
James Ward-Prowse's delivery is surprisingly disappointing as it hits Nicolas Pepe painfully on the face.
It does fall for Danny Ings, who takes a snap shot at it, but it's straight at Bernd Leno.
Southampton 1-0 Arsenal
Ryan Bertrand does well on the left to hold it up. He wins a free-kick for a push from Thomas Partey and Southampton won't rush.
Southampton 1-0 Arsenal
Nicolas Pepe is offside but the players don't know yet. Fraser Forster comes scrambling out to the back post to deal with it.
He bounces out to Eddie Nketiah and his shot is cleared off the line by James Ward-Prowse. Then the whistle goes! Scramble.
Southampton 1-0 Arsenal
Alan Hutton
Former Aston Villa defender on BBC Radio 5 Live
The most annoying thing for Mikel Arteta is waiting for his side to spring into action until about 15 minutes ago.
Southampton 1-0 Arsenal
Daniel N'Lundulu is on for Che Adams. He's worked his socks off again.
Southampton 1-0 Arsenal
Eddie Nketiah puts a good ball in. It almost falls for Nicolas Pepe. He does receive the ball a few minutes later but can't link-up with Alexandre Lacazette on the edge of the box.
It was Danny Ings who was back there defending.
#bbcfootball or text 81111 (UK only)
To those slating pepe - he's played some nice passes, beaten people, won fouls, run. Compare that to Willian and it's chalk and cheese. Nketiah is dreadful. Even his finish was going wide till it hit the defender.
Ben in Bedford
Southampton 1-0 Arsenal
Alan Hutton
Former Aston Villa defender on BBC Radio 5 Live
It's really end to end. Arsenal are trying to open it up but it will also open up opportunities for Southampton.
Southampton 1-0 Arsenal
Southampton are living on the counter-attack now. Che Adams and Stuart Armstrong try to link-up on the right but Bukayo Saka is sharp in his defending.
At the other end, Willian earns a corner with a shot blocked from Kyle Walker-Peters.
Arsenal are going for it. We have a game!
Southampton 0-1 Arsenal
Eddie Nketiah's effort was Arsenal's first on target since the 16th minute (Gabriel Martinelli).
Southampton 1-0 Arsenal
More blocked shots from Arsenal. Thomas Partey tries to come at Southampton again on the edge of the box but Stuart Armstrong gets back well.
Danny Ings then holds it up on the left.
Southampton 1-0 Arsenal
Here comes Alexandre Lacazette. He replaces Hector Bellerin. Attacking change! Cedric Soares moves over to right-back. Bukayo Saka at left-back it seems.
Southampton 1-0 Arsenal
Alan Hutton
Former Aston Villa defender on BBC Radio 5 Live
Arsenal have just started to up the tempo, they know time is running out and they have definitely upped their play.
Southampton 1-0 Arsenal
Southampton are having to do some defending now. Those changes have definitely made a difference for Arsenal.
Southampton 1-0 Arsenal
A cross comes in from the right and it bounces off Kyle Walker-Peters. It may have hit his arm but there's literally nowhere else he could have moved it - it's stuck inside to his body.
It falls for Eddie Nketiah in the box but his strike is blocked. Alexandre Lacazette is getting ready to come on...
Southampton 1-0 Arsenal
Alan Hutton
Former Aston Villa defender on BBC Radio 5 Live
What a great pass from Nicolas Pepe, a brilliant burst into the box from Eddie Nketiah but what a pass from Pepe. Best move of the match from Arsenal.
Southampton 1-0 Arsenal
Important save from Fraser Forster.
Nicolas Pepe slides it through for Eddie Nketiah and he hits it first time with the outside of his foot.
Forster is out sharply to make the block.
Southampton 1-0 Arsenal
Stuart Armstrong plays a lovely ball down the left for Che Adams to chase. He curls it across the box and it comes through to Danny Ings.
He cuts it back for Theo Walcott, who pulls off the defender in the middle, but he skews the shot over.
Southampton 1-0 Arsenal
Thomas Partey has a go from way out despite having options either side. It's miles over.
#bbcfootball or text 81111 (UK only)
Minhaz Uddin: Why take off Martinelli?? Everybody wants to see William to come off for saka.
Paul D White: Last season we only qualified for the Europa League because we won the FA Cup. Our league form is just as bad this season, and we can’t afford to lose European football revenue.