This happens to be the main factor that anybody hasn't copied GT4 on PS2 succesfully Are you sure about that, all dual layer pressed disks are ptp as far as i know, just use dvd decrypter to backup any ps2 disk and it should work fine. Gta4 is only the Jap version at the moment, so your no doubt meaning the warez version that people cant get to work right or at all, which is mention on fourms elsewhere, it may not be the disk as most older mod chips dont support dual disks. Best thing to do is buy game when it comes out and you'll find that it probably backs up fine, direct from the original.
Clever, isn't it? No, far from it