GTA 4 is a very poor port of a console game. A quad core CPU is recommended, dual core CPU is considered minimum, and a single core CPU means you are out of luck. That means the game is very CPU limited.
Back when the game came out, I remember reading complaint from someone who had a powerful quad core CPU (Q9650??) and 8800GTX video card. Standing still he only managed to get 20 FPS @ 1920 x 1200 resolution.