Hammers opened scores after five minutes, Michail Antonio heading home unopposed at a corner, before the away side made it 2-0 on 12 minutes through Lucas Paqueta.
Declan Rice crashed home a third from another corner minutes before the break, substitute Pablo Fornals adding a fourth in the second period.
Talking to Sky Sports,
Chris Mepham shared: “Weird game really. I felt like we probably felt in complete control, had a lot of the ball, started the game well and then just set pieces killed us. It’s something we’ve put a lot of work into and we’ve been aware we’re not good enough dealing with them set pieces and today showed again – two balls in the box and two goals and you’ve got a mountain to climb against a very stubborn West Ham team. We need to do more to make sure we’re not in that position but I felt like that was the difference today. I don’t think in general play there was much in it either way. Just shows the importance of set pieces at this level.”