How scratched (or burned) is the disc?
If it's not at all readable, then obviously, there's nothing you can do but get a new copy of the game.
My local
Game shop do a service where they take/add (can't remember which, probably add) a new layer onto the disc which can then be read by the Xbox.
If it's GTA IV then it's likely that they'd have a copy they can repair it with. It costs, but it's cheaper than a new disc.