According to Gamespot via GamingExcellence, Rockstar has suggested that the fourth sequel in the GTA series will support online multiplayer with up to 16 players.
Word came from the developer's representative at the Playstation Holiday Preview Event in Toronto. "The developers were considering the possibility of having an open world completely unlocked from the start of the game," "[Rockstar] is aiming for at least 16 simultaneous players, with more than just usual deathmatch mode."
We are talking about a rumor and nothing is certainyet, but this might be the reason why the game was pushed back to Q2 2008. Rockstar, as usual, didn't want to comment on rumors or speculation, so we will just have to wait and see.