OK for example when they performed thir latest hits on Later with Jools Holland, they're definitely a foursome, and I think in the 'Wake Me Up When September Ends' video they were also a four piece.
I hate Greenday too much, but fortunately I know the reason..
Basically, all recordings are done by only the trio, but when playing live some songs are blatantly impossible for only three band members to play. So for songs that include two guitars, they have this guy.. I forget his name.. that plays them live with the band.