There is no such thing as a MacBook G4. If it says iBook G4 on the screen, that is the name of the computer itself. And that's not to be confused with the name iBook given to the iOS and Mavericks software for reading Apple published electronic books. Apple when they changed to the Intel chip renamed their consumer machines MacBook. Now the line between MacBook, MacBook Pro, and MacBook Air is more blurred as the 13" MacBook Pro has many of the features more expected of a consumer machine than a Pro machine.
iBook G4's with a minimum of 867 Mhz can handle Mac OS X 10.5 retail. Otherwise they need Mac OS X 10.4.6 retail or earlier. If they didn't have the original discs, you won't be able to run the old Mac OS 9 from 2001 on them.