I love both GTA IV and GTA San Andreas. But both games are nothing alike.
I love GTA IV, but I have always felt confined and limited in terms of exploration. GTA IV seems to focus more on the details of the city, and allows the layer to make choices during missions. But in GTA San Andreas, your character feels more personalized. From haircuts, clothing, body shape/size and weight. In GTA San Andreas, you were way less social than you are in GTA IV. GTA IV focuses more on the social aspect rather than the exploration aspect. GTA San Andreas was all about living the dream, buying as many houses as you can, personalizing your vehicles with rims/paintjobs,etc., exploring mountains/forests/deserts, discovering new things! Plus, GTA IV has a more modern touch, yet San Andreas feels more like a 90's gang vibe (which isn't bad at all).
Basically, it's "Social vs. Exploration".
But, in a couple months, both GTA San Andreas and GTA IV will have a baby, and create GTA V! Cannot wait! :D
Tell me what you think, GTA SA or GTA IV?