West Ham, who are moving to London's Olympic Stadium next season, after 112 years at Upton Park's Boleyn Ground, were guilty of slack defending as Swansea took a 2-0 lead with goals from Wayne Routledge on 25 minutes and Andre Ayew after half an hour.
Ki Sung-yeung starred in a makeshift Swansea side shorn of key players like Ashley Williams and Gylfi Sigurdsson, scoring Swansea's third at the start of the second half.
Diafra Sakho scored from close range for West Ham on 67 minutes but substitute Bafetimbi Gomis, again on the break, made it four for Swansea in the final moments.
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(Reporting by Richard Mably; Editing by Toby Davis)
