The REAL Big four, unlike someones definition, are the kings of thrash metal.

Intelligence agency term for "psychological operation". A government or corporate-sponsored operation, usually taking the form of a "terrorist attack" or "crazed gunman on a spree", with the intent of panicking the public into demanding more police and laws inhibiting freedom. Psyops are usually carried out by drugging a civilian or group of civilians with aggression-promoting drugs, psyching them up, arming them, and sending them out to commit mayhem. Government-sponsored terrorism. See also blackshirts, conspiracy

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

There are four essential items in any mans life: pound the pussy, eat some food, good shit, and a nice nap. Pound. Eat. Shit. Nap.

The big four is when you can hit all those items in a single day. But it only counts when you hit all four. You can't count two eats, one shit, and one nap, as a big four. However, once the four are completed in a single day, you can add on additional items for a Big Five, Big Six, and so on.

The Big Four are the largest railroad companies in the United States. The Big Four consists of CSX, BNSF, Union Pacific and Norfolk Southern. Kansas City Southern almost makes it in, but the foamers would never allow it.