Apple has introduced a firmware update for recent Power Mac G4s. It is available using the Software Update feature.
The version 4.4.8 update is for Power Macs purchased since August. These models have mirrors on the optical drive doors, and contain DDR RAM; and include the multiprocessor Power Macs introduced then.
Apple's own notes say this update improves the efficiency of the internal fan in these models. It reduces fan-speed cycling when users run Mac OS 9, and improves fan speed.