This is why I specified using "DVD-R" in my previous post about burning the disc. This could almost certainly be your problem.
I would've bet my life that I've owned DVD-R DLs, but I can't find any trace of them online so maybe I'm mistaken.
I am surprised that the iMac G4 won't read +Rs though, so far the only thing I've found that doesn't like +Rs are computers back to the iMac G3 early graphite G4 era. iMac G4s are rather new by those standards.
DVD-R DL do exist, they were just rarer than DVD+R DL discs. I have a few still-unburned ones on a spool right next to me. (Along with a few rare-as-hens-teeth DVD-RW DL discs I've reused a few times. Although I'll be damned if I can remember which optical drive I own actually supports burning those - I know none of my Blu-ray drives support it...)