I finally got an imported copy of chinese GT4 tonight, and I knew it won't boot directly on my US PS2, so first I make an ISO file out of the disc with DVDDecrypter on my PC, and load it on a harddisk using WinHIIP 1.6.5(Using Mode 3, refer to this list: hxxp://homepage.ntlworld.com/casey.miller/HD%20Loader%20Compatibility.htm). With my harddisk installed using Network adapter (of course), I boot up with HDLoader v9 Patched (hxxp://srlz.altervista.org/software/software.html). The game is listed there but if I try to load it directly, it will go black screen after the disclaimer, SO, here's the trick: before I choose the "Play" menu in HDLoader, I put the GT4 disc back in the PS2 and vala! The game now run, but too bad it's not off the harddisk but from the GT4 disc. I played thru B-Licence, a few cups, with GT Force and GT Force Pro, no problem seen so far.
Note: You can't directly copy the game from DVD to harddisk with WinHIIP, it will only copy the first layer (3G or so), with DVDDecrypter, you will get around 5.8G.
Note: You can get WinHIIP 1.6.5 here: (hxxp://forums.psxforum.com/viewtopic.php?t=22748)
Note: My PS2 is modded(but still unable to play import!?! I have no idea), so YMMV, but I read somewhere that it doesn't matter if you have mod chip or not if only loading installed game on harddisk.