I think it depends on the model of G4
Yep, no problem with 500GB HDs on my PowerMac G4 933.ya, "older (Pre-Mirrored Door) G4 and G3 Macintoshes -- including Cubes, PowerMacs, PowerBooks, iBooks eMacs and iMacs" "Later model G4's and all G5's and Intel Macs do not suffer from this limitation."
Does the power mac g4 have the 128 gb hard drive limit? if it does how do i get around it