I thought it was a DivX derivative.
other way around, DivX is a variation of microsoft's premature mpeg 4 codec (msmpeg4 version 3). Anyway mpeg 4 is an mpeg standard, just like mpeg 1 and 2, but mpeg 4 is more or less brand new. DivX 4, 5, and XviD are all ISO compliant mpeg 4 codecs. Anyway, I'ma guess you're talking about the mp4 file format, cuz you probably downloaded bone's Lil Piece o' Heaven video you sick bastard (jk, that video amuses me >_<) in which case...
you need wmp4player to play this, find it on doom9's software page