I have a PowerBook G4/500 (2nd version) with 512 megs of RAM running OS X.3.9.
Lately, it just doesn't want to shut all the way down. It gets all the way to the blue screen and the spinning/fading clock, but hangs there.
I have to force quit it with Command-Option-Escape-Power.
It will do a Restart fine, but not a Shutdown.