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1. Big Four
The REAL Big four, unlike someones definition, are the kings of thrash metal.
The big four are:
Metallica
Megadeth
Anthrax
Slayer
In that order
2. Big Four
Usually relates to the "Big Four" Grunge bands of the early '90s.
The Big Four are:

Soundgarden
Pearl Jam
Alice in Chains
Nirvana

Even more interesting, they all come from Seattle
"The Big Four were pioneers for the Grunge genre, not to mention, they just plain kick ass!"
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