There are a few possible cures. I encountered the same problem with my slim PS2 last summer - it seems to be partially heat-related on the slim model. If like most people you have the fat PS2 though, you may be experiencing an early symptom of Disc Read Error, which will eventually require either a realignment of the existing laser, or a new laser altogether - GT4 seems to be the first game that quits working when DRE begins to occur.
Before you go down that avenue though, try cleaning the lens with a q-tip lightly moistened with a well-diluted solution of water and rubbing alcohol. Even after this, it may occasionally take several attempts for the PS2 to read the disc, as GT4 is probably the single most data-intensive disc ever manufactured for the PS2.
If you have a slim PS2, keep it as clean and dust-free as possible, and keep it in a cool area - definitely do not leave it sitting on top of another operating electronic device, and don't set it on carpet. If possible, have a small fan pointed at it during operation, and don't leave it on for long periods of time. The fat PS2 is fine with that - the slim isn't. Good luck!