i think the reason gta4 felt so stripped down compared to san adreas was because it was the first in a new 'set' of GTA games, meaning it was the first on a new generation of consoles and had to be built from the ground up - new engine and all
in the same way, gta3 was much shorter on features compared to san andreas, because it was the first in that generation of GTA games
i have a feeling that if they make two more GTAs for this console generation they will eventually match up to the style and content of san andreas