Grand Theft Auto 4 does not have split screen mulitplayer.

[ToastyJaguar]

"The multiplayer modes cannot be played split-screen, so forget about playing this with buddies on a single couch. The modes are online only and identical for the PS3 and Xbox 360 versions, according to Rockstar reps."

If they have gone to R* and heard this from their mouths then it should hold some truth to it.

http://www.mtv.com/news/articles/1585030/20080408/index.jhtml?rsspartner=rssColdFusion
for the original article