Well, not really. They'd really hit the limit of the game engine, and there wasn't much left to do that wouldn't just look like the same old thing all over again. GTA4 was an incredible accomplishment, but I must say that I still prefer the GTA3 series.
I love all of the GTA III era games.
I don't see why they had to have GTA IV take place outside of it. They already had such a great universe going. They could have just kept building on it, and making it grow bigger and bigger.
I didn't love them all. I enjoyed 3, SA and LCS.