Match ends, West Ham United 0, West Bromwich Albion 1.
Slaven Bilic enjoyed a winning return to London Stadium as a dismal performance by West Ham saw them crash out of the FA Cup at the hands of his West Brom side.
The Croat, facing the Hammers for the first time since they sacked him 2017, told Match of the Day afterwards: "Every time you come back to a place you have so many good emotions about, of course you want to do well.
"So it is great for us, for me personally and for Dicksy [West Ham cult hero Julian Dicks, his assistant].
"Did it feel a bit sweeter to win here? Yes, a bit to be fair, but it is not that I am sour or that I had a point to prove here. I just wanted to win.
"We big-time deserved it. It was a great performance by us - a complete one."
Conor Townsend's rasping early strike proved enough to send the Baggies through to the fifth round, although the Championship leaders had enough chances to win by a much bigger margin.
West Ham offered little in response, even after West Brom had defender Semi Ajayi sent off for his second booking with 18 minutes remaining.
Hammers boss David Moyes had named a strong line-up but his side took until the 90th minute - when Declan Rice tested Jonathan Bond from distance - to muster a shot on target,
In contrast, West Brom were a constant threat while they had 11 men, with Charlie Austin twice going close and Kyle Edwards also testing Darren Randolph.
Bilic had made it clear before the game that promotion is his priority this season - but his side go into the hat for Monday's draw.
Hammers left to focus on battle for survival
Bilic made eight changes from the team that lost at home to Stoke on Monday, his side's latest slip-up in a run that has seen them fail to win in the league since 14 December.
But the visitors were still too strong for a disjointed West Ham side who repeatedly gave the ball away in the early stages.
The hosts had plenty of chances to clear the danger in the lead up to West Brom's goal but Carlos Sanchez's clearance cannoned off Issa Diop into the path of Townsend, who had time to take a touch and pick his spot.
After West Ham were booed off at the break, Moyes made a triple half-time substitution, bringing on Michail Antonio, Angelo Ogbonna and Mark Noble.
"We did not start the game well," Moyes said. "We gave up opportunities in the early part of the game and they were better than us in the opening period and they got their goal in that time.
"We were a bit fortunate we did not concede another goal but we got through to half-time and I made a decision to make a few changes.
"I had seen too many poor passes, too many balls being kicked off the pitch and not enough good play. We had to attempt to get through or at least get a draw but we were not quite able to get there."
Antonio at least gave the West Brom defence something to worry about with his runs down either flank, but their best chance of an equaliser came and went when Noble blazed over when well placed in the final moments.
Defeat means the Hammers have only a battle for survival at the bottom of the Premier League to look forward to, and this was not a performance that will give their fans much optimism that they can beat the drop.
Man of the match - Filip Krovinovic (West Brom)
What next? A big week for West Ham
This is a big week for West Ham and it has got off to the worst possible start.
The Hammers, who are only above the drop zone on goal difference, are back in Premier League action when they host leaders Liverpool on Wednesday (19:45 GMT) in a game that was postponed before Christmas because the Reds were playing in the Club World Cup.
Then Moyes' side, who have won only one of their past eight home league games, are back at London Stadium to play relegation rivals Brighton next Saturday (15:00).
West Brom, meanwhile, must hope they can carry their FA Cup form into the Championship as they look to end their wait for a league win at Cardiff on Tuesday (19:45) or at home to Luton on Saturday (15:00).
- 4BalbuenaSubstituted forOgbonnaat 45'minutes
- 23DiopBooked at 49mins
- 15C SánchezSubstituted forNobleat 45'minutes
- 18FornalsSubstituted forAntonioat 45'minutes
- 20Soares Cardoso
- 6AjayiBooked at 72mins
- 10PhillipsSubstituted forTullochat 62'minutes
- 18BarryBooked at 39mins
- 7KrovinovicSubstituted forBartleyat 74'minutes
- 15AustinSubstituted forZohoreat 70'minutes
- 25Harib Al Habsi
- Stuart Attwell
- Shots on Target
Second Half ends, West Ham United 0, West Bromwich Albion 1.
Attempt missed. Mark Noble (West Ham United) right footed shot from the centre of the box is just a bit too high.
VAR Decision: No Penalty West Ham United.
Foul by Angelo Ogbonna (West Ham United).
Kenneth Zohore (West Bromwich Albion) wins a free kick in the attacking half.
Attempt saved. Declan Rice (West Ham United) right footed shot from outside the box is saved in the top centre of the goal.
Attempt saved. Declan Rice (West Ham United) right footed shot from outside the box is saved in the bottom left corner. Assisted by Manuel Lanzini.
Attempt blocked. Rayhaan Tulloch (West Bromwich Albion) left footed shot from the centre of the box is blocked. Assisted by Kenneth Zohore with a cross.
Attempt missed. Pablo Zabaleta (West Ham United) header from the centre of the box is just a bit too high. Assisted by Manuel Lanzini with a cross.
Attempt missed. Manuel Lanzini (West Ham United) right footed shot from outside the box misses to the left. Assisted by Angelo Ogbonna.
Attempt saved. Issa Diop (West Ham United) with an attempt from the centre of the box is saved in the top centre of the goal. Assisted by Mark Noble.
Corner, West Ham United. Conceded by Gareth Barry.
Attempt blocked. Michail Antonio (West Ham United) right footed shot from outside the box is blocked. Assisted by Declan Rice.
Foul by Declan Rice (West Ham United).
Kenneth Zohore (West Bromwich Albion) wins a free kick in the defensive half.
Foul by Sébastien Haller (West Ham United).
Conor Townsend (West Bromwich Albion) wins a free kick in the defensive half.
Attempt missed. Angelo Ogbonna (West Ham United) left footed shot from outside the box is close, but misses to the right. Assisted by Manuel Lanzini.
Attempt missed. Mark Noble (West Ham United) right footed shot from outside the box is close, but misses to the right. Assisted by Manuel Lanzini.