GTA 4 lacks major variety compared to San Andreas. For example: Can't fly planes, can't drive combines, can't have a jetpack, no driving school, not much to the environment, no bicycles, no parachutes, no different fighting styles, no flamethrower, no stats.
GTA 4's physics are totaly awesome though. The skeletal physics (improved version of ragdoll phyics) are fun to use when using a car on a pedestrian, and the car body crash modifications are grand, too. I can smash a car against a wall and make it skinny, or fall on the roof of it and cave it in.
Bull shit.
I personally like all the GTA games,but I would most definitely like a GTA that uses the physics of GTAIV with the freedom of San Andreas.