I don't really know what to expect from GTA4 to be honest ... If you look at the line-up of the most important games on the Playstation and compare it to the most important games on the PS2 you may understand what I am thinking.
The Playstation had several games on it which were so popular that they spawned a genre and became the cornerstone of the generation; games like Resident Evil, Tomb Raider and Tony Hawk really were remarkably important to the Playstation's popularity. The unfortunate problem was that the publishers of these games wanted to cash in on their success so they cranked out as many versions of these games as was possible; their competition also was releasing clones at an amazingly rapid rate.
Eventually, people became tired of these games and the genre as a whole shrank and only the original games remained (and even they had lower sales). When you look at the Sandbox Crime genre you see a very similar pattern and I could see that at some point people just stop paying attention.
