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