I've recently been working out how to play GT4 over LAN on PlayStation 2. Turns out it's not difficult at all and there is a great site about it here. One thing that did take some time to troubleshoot however, is that one of the discs I had didn't work in LAN mode. The symptoms were that just after the 'waiting for connection screen', with the 1-3 dots at the bottom, the machine running the suspect disc displayed a black screen. Music continues to play and, to the other players, the machine appears to be connected but the machine is unresponsive. These symptoms occured on any machine that had the suspect disc in it (I tested on PS2 fat and slim consoles). Everything worked fine with other discs. The suss disc was almost perfect except for one scratch (more of a scuff really). I polished the scuff out with brasso until the disc was in much better shape than some of the other discs I tried but it didn't fix the LAN mode problem. I briefly went into GT mode and Arcade mode and the game didn't show any other issues.