the update once done works for OS X and OS 9 but per firmware says must boot into OS9 to update firmware and such.
NOTE: If the Boot ROM Version is earlier than 4.1.9, reboot the system under OS 9. The Apple Power Mac G4 Firmware Update will run only On Power Mac G4 models with AGP graphics running Mac OS 9.1 or later from a local hard disk. If you are using Mac OS X on your Power Mac G4, you must start up from a local Mac OS 9.1 or later writeable partition (not a CD or network disk) before following the update instructions.