The big four of thrash has widely been considered to be Metallica, Megadeth, Anthrax, and Slayer. They've even toured under the moniker as one big package. This has also been up for debate over the years, as some people have felt like influential bands have been shorted in their recognition.
Instead of asking who should be replaced, Twin Musix asks Anthrax bassist Frank Bello who he'd add as the fifth official name in the thrash icon group. He says Testament and Overkill certainly deserve a spot in the listing, but he's going with Exodus on this one.
Exodus, because we have been there from the very beginning. It is hard because we have all been there for a really long time. Testament, Overkill and there is a lot of great bands.
Personally, I agree. Especially considering all Gary Holt has given us over the past 10 years, in terms of Slayer and killing the Kardashians.