Dude buy the game, youll get caught playing a pirated copy of GTA4 and will miss out on the multiplayer experience.
Plus will need to go through the hassle of opening your xbox360, connecting the dvd drive to your computer (Even then you need to have to correct sata chipset), modify the firmware on the dvd drive which isnt as easy as it looks, then put it back together. Then youll need to "stealth" the image then burn it to a specific brand of Dual layer DVD.
If you havent got the specific equipment you will probably end up spending more on that than if you bought the game.