Let us know your hardware and software configs:
1. Which Mac model in detail? (Don't just say "it's a G4"....there have been MANY models with a G4 CPU as well as other CPUs.)
2. Which version of Mac OS X do you have installed currently? (Be specific. Head to the Apple menu, select About This Mac, and provide the version number from that window on here.)
3. Provide all of the procedure you tried in order to update. You may need to download the Combo Updater of the latest version of whichever revision of OS X you're running.
With more information, we can help you better, so please be specific.