What graphics setup are you running? I realise it's onboard from your post (as you state you don't have a graphics card) but what kind of motherboard are you running that is producing your graphics?
First impression, no you won't be able to play GTA4 on on-board graphics. Although it's very heavily CPU dependant, it does still need some (fairly) heavy input from the Graphics Card so I would've said you WILL need one. That said, you say Far Cry 2 runs on your system? What resolution and graphics settings are running FC2 on, and what settings do you hope to run GTA4 on?
Sorry for all the questions, but give us a little more info and someone'll come up with a definite yes or no, I'm sure.
Also, have you tried "Can I Run It" - http://cyri.systemrequirementslab.com/srtest/
You can select GTA4 as one of the drop downs and it'll tell you how your PC compares to the minimum and recommended specs for the game.