It will probably have an online feature, as said here:

Multiplayer
Multiplayer will be a feature of GTA IV but will not be an MMO, as noted by Dan Houser. It will be accessible from Niko's mobile phone and is aiming for up to 16 players, with a possibility of open-ended gameplay and exploring the whole city.

Later revealed in an issue of PSM, GTA IV will have 15 modes of play and support up to 16 players, with the player using a customizable character to play as, with a choice of gender, race and clothing. The modes of play include several varieties of deathmatch and racing, with a cooperative single player mission and free mode. Other new multiplayer modes include Mafia, in which players try to assassinate a target before their opponents can, and Cops 'N' Crooks, in which one team of police officers must stop the other team of criminals from escaping. The map used in multiplayer will be the same as the single player map and the host of the games can control many variables like traffic, police reaction and laps in races. Rockstar has left one game type unknown and not explained some.

That was from wikipedia.org