I would not buy it cause I thought GTA4 was garbage lol They'd have to either remove or add so much extra to the game to make it worth it. I hated the friend stuff, the physics I wasn't liking, they removed a lot of the customization stuff, it's been the only GTA game since 3 I actively hated. If they could change tons of that, maybe...but I'd rather they not work on it.
The better game to remake if they aren't making a sequel is Red Dead Redemption. Give us that with overhauled graphics, a bunch of extras, and whatever other goodies they could give and I'd buy that in a heartbeat.