I'll definatly still be picking GTA4 up because SA was probably the best game i've ever played.
I wouldn't get my hopes up to much, GTA4 is basically just the a story line and a thin shell around it. Only reason why it got so good review scores is because R* shipped the game to the reviewers 2 days before release date, so no reviewer managed to play the entire game and notice all the things that are lacking before they had to review the game.