I'm sorry for resurrecting an older thread, but have you tried MorphOS on your Power Mac G4? It's an exotic, alternative OS for select PPC hardware and it isn't based on Unix/Linux/BSD. You can download the Live CD ISO on the official website, you should be able to burn it to a CD and boot from it to test the OS. If you like it you can install it on your hard drive. Get it here: http://www.morphos-team.net/downloads.html