Couple of options to try:
1) Reboot system and hold down the trackpad button (under the track-pad) during boot-up this should cause the drive to eject anything still left in the drive...
Option 2 (Not sure if it will work on the older G4's)
Eject a disc with Open Firmware commands
Follow these steps:
Restart the computer.
Immediately after the startup sound, press and hold the key combination Command-Option-O-F.
Release the keys when you see a white screen that says "Welcome to Open Firmware."
At the prompt, type: eject cd
Press Return, then wait a few seconds. The disc drive should eject any disc that is present, and "ok" appears behind your command when the action is complete.
Hope this helps :)