Metallica, Megadeth, Anthrax, Exodus (not Slayer). As much as we might want it, Slayer was never considered one of
the original big four (probably should have been a "big five"). I have seen Slayer replacing Exodus on several pages now including this one and Wikipedia. Not so. Slayer was often categorized differently than the mainstream 80's Thrash bands (
Death metal, etc, although
I don't think this exactly fits either).