I'm not sure about Anthrax belonging in the top 4 due to popularity alone. Slayer would contradict that logic. No music videos, way too aggressive for daytime radio. Don't get me wrong, metal heads knew about Slayer, but EVERYONE and their uncle knew about the other 3.
Thrash was in the limelight. Sooner or later, if a band was making waves, it would have been noticed. The band could let the music do all the talking. Outside of the big 4, the next tier category would include: Suicidal Tendencies, Testament, Voivod, and maybe Exodus.
After that, you're talking Flotsam, Forbidden.....indeed, it takes a while before Overkill would even fall in line. I'm not debating their strengths, but fuck me, considering just how "heavy metal" they were, blending speed, thrash, and traditional all in one, together with a great live show and frontman, it's amazing that they didn't make it 'bigger' (inside metal circles and out).