latest OS that I can run on an iBook G4?

Depending on the G4's specs, it may be able to run Mac OS X 10.5 Leopard. That's definitely the last version that would run on any iBook G4. You can see the system requirements here:

http://support.apple.com/kb/HT3759

If your iBook doesn't meet those specs, then 10.4 Tiger would be the most recent you could run.

Regards.

Need more info

Mac OS X 10.5. If it's CPU speed is below 867MHz, click here.

(63449)