Yeah the disc should be burned in UDF... I think the reason it's not working from DVD+DL is because none of the backup solutions seem to support discs with multiple data tracks (correct me if I'm wrong)
So, basically, this is what the GT4 disc looks like:
Layer 1, Track 1, UDF:
system.cnf
gt4.vol
core.gt4
scaj_300.07
ioprp300.img
epson\*.*
irx\*.*
net\*.*
Layer 2, Track 1, UDF:
folder?\system.cnf
gt4l1.vol (this filename is verified)
Isobuster reports that the system.cnf on the 2nd layer is placed in a folder. The system.cnf file on the 2nd layer is identical to the one on the first layer.
We'll have to wait and see what HDAdvance does with the gt4l1.vol file included this time, but my guess is that the only way of getting GT4 to work on a backup solution is by ripping files from the .vol files and make the game fit on a DVD5.
Editing the .vol files might be possible if the GT4FS (filesystem) will be documented and a tool is written. I know there already is a tool to extract files from the GT3 .vol files, so we might see this soon