Marlon Harewood's unforced penalty, Luke McCormick's soft own goal and Malky Mackay's tap-in meant it was game over before Teddy Sheringham's late second-half double rounded off the Hammers rout.
"We didn't do ourselves justice and I'm disappointed for the fans who turned up in good voice and good number," the Argyle boss said after the game.
"If you analyse all their goals we could have dealt with them better.
"It was our worst performance as a team this season but if we got a goal back we could have asked questions of West Ham and they might have got nervous."