It's hard to say. I guess it all depends on when the next gen systems will actually be out. They already have a really good game engine to work with, so it's all about pumping out new assets like they did with Vice City and San Andreas. Vice City only took a year to make, so it would seem likely that a GTAIV sequel would take two and a half or three years.
If they could get a new GTA game out by the end of this console generation, it will be huge given what the install base for each system will be by then. Guess we'll just have to wait and see.