Seems that error means the PS3 is having trouble reading discs. Have you tried the GTA4 disc in a different PS3?
Couple of threads you should take a look at : Discs are not loading in my PlayStation 3? - PlayStation Forum & [FAQ] Blu-Ray Drive Fail - PlayStation Forum
i dont think its the Drive i just replace the disk drive and lens about a month ago and every sinlge game and movie works except for this one i played the game fine on my brothers ps3
Check that there are no scratches, smears or smudges on the disc. That can cause disc read errors. Also, take a look at the side of the disc. Sometimes hairline cracks in the disc can cause issues but aren't visible if you look at either of the flat sides of the game.