EDIT**
Here this should help mate
1, Rip GTA4 to your HDD with multiMAN,
2, Once finished, Back out of multiMAN & then go back in to multiMAN (Just for it to refresh & show your installed GTA4)
3, Take out the disc & replace it with some other random PS3 game,
4, Still in multiMAN, Hit X to load GTA4, Let it fix permissions & Say NO to direct boot,
5, On the dashboard, Select the disc icon & hit X (It should say GTA4 on the disc icon even though you put some other random game in the BD drive).
6, It should load.
I've done that a lot of times! But the way that I fixed this was by putting the game into my EXTERNAL HDD because for some reason GTA IV wont work in interal