Only if you were to try and use it as a boot drive. If you were to use it as a secondary drive, no need to change the partition table format at all.Although, if you were to use it on an Intel Mac you would have to change the partition.
It was just that I read the G5 uses a Serial ATA drive - see
http://www.everymac.com/systems/apple/powermac_g5/stats/powermac_g5_1.6.html whereas the G4 disc is IDE, although it says it's IDE ATA. Is that the same as Serial ATA? All these abbreviations are most confusing. Maybe it has additional slots for the G4's drive?