I liked this post, so I went ahead and tried it for you. I have a Driving force Pro, but yours should work too. GTA4 is pretty much unplayable with this wheel. On foot, there's no way to make Niko walk or run (Since the pad controls the radio stations and your cell phone), and turning the wheel makes him spin in place. So, I grabbed the controller and carjacked an old lady, then switched the DFP to controller slot 1. The pedals don't work, so that means you'll have to accelerate and brake using the X and [] buttons; the steering works... Which doens't sound too bad, if it wasn't because of the AWFUL dead spots. The dead spot is HUGE and turning the car around is real troublesome.
So, in a nutshell, to succesfully play GTA4 with a Driving Force Pro, you'd need to:
a) Use the in-game XMB to switch your Sixaxis controller to slot 1 everytime you get out of a car to control Niko and map the DFP to controller slot 1 every time you get on a car.
b) Get used to the button mapping of the game while in-car; there's no custom configuration for the game's controls, so your choices are to use X [] or L2 R2 for gas/brake.
c) Get used to the huge dead spot of the wheel.