Tell us your PC Specs. Do you have any graphic mods installed (ENB, Ultimate texture tweaks, and any 3rd party graphic enhancer). Also, how much VRAM does your GPU have (GPU memory). If your card has 512MB or less and you're playing on High texture resolution and screen resolution, this could be the problem. I had a Hd4850 with 512MB and i had to play on low settings.
Also, if you overclocked your video card, put it back to stock because that may be the reason why you're getting texture defects and artifacts.
Try playing a different, system hungry game (like Crysis, Metro 2033, BF3) and see if your getting artifacts/texture defects.
If its only with GTA IV, reinstall the game.