GTA IV and SA--
Is not just a game, it is living your character's life where you can make him do many things apart from the main mission, like robbing, getting into unnecessary fights with people, stealing vehicles, doing accident, pubing, flying planes or choppers, going to pros, eating, drinking, shopping, getting new hair cuts, visiting new places as tourist or as a vagabond, dating, f***ing, swimming, racing, surfing internet(inside the game they hv their own internet, I wish in future games somehow it can be connected to the real one), helping people, .............................. I can not type more then this.
In one word, this is the best sandbox game. What is sandbox, wikki it.
GTA IV vs SA --
Off all the above things, 1/3 of them you can not do in GTA IV though it is the latest version series, not sure why. So in that respect I still play SA sometimes.
Rest, GTA IV has extremely , did I say extremely .. oh yes I said extremely .. cool graphics and game play such that if you do not have a Quad and a good card, do not even try it.
I am 32 years , Manager in my firm, and this game helps me live my life and get me quick relaxations in short spans of breaks. For this game I sold my Core 2 Duo, bought i3, sold it the same week, got a hexacore and recently got GTX 460.
Most of the time I do not even play missions, I just simply make the character do all those a above things and as everything gets noted and improves the character during game play, it makes me feel if that person is aging along with me.