If you're hoping to play a demo of GTA 4 and try it before you buy, then you'll be mighty disappointed. Demo versions will not be here before the official release and it's still uncertain whether they'll appear afterwards.
Take -wo and Rockstar Games decided on such a move due to the complexity of the game and actual deadlines. They also said that this short period of time until the release (29th of April) should be spent in fine-tuning of the game, something they consider more important than the demo.
Those of you that don’t care about the demo can hop over to the official website and check out the rest of the content – trailers, additional info and such.