From Bulbapedia, the community-driven Pokémon encyclopedia.
The original Big Four forums were the boards which had formed the "Anti-Spammer Alliance" from 1999-2000, and were generally considered to be the largest, most active, and best ran Pokémon forums of their day. They were, in the order presented by the original banners, Universal Pokémon Network, Bulbagarden, The PokéMasters, and Pokémon Abode.
The use of the term Big Four to refer to these sites and this era in the fandom did not begin until after such time as all the constituent sites of this group had died. It is believed to have begun only after February 2002, with the term most likely having been coined by Archaic. With the exception of Pokémon Abode, all of the Big Four have since been resurrected; however, of the three surviving sites, only The PokéMasters is currently run by its original webmaster.