Don't know whether this will work with your particular problem, but I've had problems with both shutting down and restarting my G4, too, although it's better in Panther than it was in Jaguar, which would never shut down at all. Having scoured these forums for help, I no longer even attempt to do it via the Apple menu, in case it doesn't work I always use the terminal. Open the terminal (Applications/Utilities/) and type in either
sudo shutdown -h now
or
sudo shutdown -r now
(shutdown, or restart, obviously).
Then on the prompt type in your administrator password.
Saves the wear and tear that hard shutdowns can cause and is less frustrating than using the Apple menu and keeping your fingers crossed.