Complex sentence: The game is played by 2,3 or 4 players, but the 4-player version is considered to be the real game, and other two are just crippled versions played only if there aren't enough players available.
Split into simpler sentences: The game is played by 2,3 or 4 players. The 4-player version is considered to be the standard game, and other two are just crippled versions played only if there aren't enough players available.