GTA 4 is going to be a game in itself. Rockstar are mature enough to make another game, rather than resorting to shoving SA, VC and GTAIII on one disk. I dont think people should underestimate R*. Why would they need to put their three old games in? I'm pretty sure they'll surprise us in the long run.
Nobody was expecting GTAIII to be so radially different to GTA2. Nobody was expecting SA to cover an entire state. Nobody was expecting Vice City to be so lame, yet so popular (i love it , actually). The only thing i dont really like about Vice City is the way half the buildings and textures are straight copies of GTAIII, and pasted badly (especially in the airport).
Rockstar will once again astoud us with their innovative use of new and old technology, maybe there will be support for the eye-toy, or three-dimensional holo-screens. yea okay, i'm dreaming a bit there, but 53gig on a disk is pretty amazing, so why can't we accept we don't have a clue whats ahead.
Maybe some of these ideas will go in, but we can't presume that it will be anything like the GTAIII series, it will be completely different. Hey sure, we can hope, but we'll be miles away, probably literally. In between the PC releases of Vice City and San Andreas, we've seen Media Centre PCs, 3GHz CPUs, desktops with 4gig RAM, terabyte hard drives and a Royal Wedding. You really can't expect GTA Four to be something we can imagine now, which in itself is impossible to imagine. Maybe we can hope, but really, we will be surprised.
Don't get me wrong, i'm not doubting the legitimacy of this topic, i just think many people are being naïve as to what R* have in store. Dont expect more of the same, expect something many, many times better.