The fan keeps running even when the computer is in sleep.
Solution
As the Power Mac G4 Read Me file notes, some PCI cards are not compatible with low-power mode.
The ATI RAGE 128 PCI card option, available only as a configure-to-order option from The Apple Store, prevents the computer from entering low-power mode when it is in sleep. The system fan continues to run while the computer is in sleep mode.
No upgrade is available to allow this card to enter low-power mode.