From what I hear, GTAIV was just a horribly made port. I've read reports of people with awesome computers having a lot of trouble getting the bloody thing to run satisfactorily. What are your computer stats?
Like, whatever mid-range was circa 2007. I mean, I would've put up with ass-terrible graphics if it'd just run well (it still looked like a top-tier Xbox 360 game, even with graphical settings on their lowest, which kind of ****ed me off).
As a kind of benchmark, my PC can run Crysis very smoothly, with the graphical quality of HL2 (which is to say, not bad).