That is typical performance in that game. As said above, GTA IV is a very badly ported game, all you can really hope for is that your framerate will stay above 30FPS, and thus the game will remain at least playable. I've run the game on high at 1080p with a 6870 and a 7870, and performance was fairly similar, out in the city the framerate will hover in the 30s or low 40s if you're lucky. The only places where you will get close to 60FPS is in confined indoor areas. Despite being 4 years old, 60 FPS solid in GTA IV is impossible, it doesn't matter what hardware you have, the game is not going to run great.