Sorry, there's no such thing as a MacBook G4. It's either a MacBook, or an iBook G4. I presume it's an iBook since you mentioned the G4.

iBook G4s range from the 1st gen marble iBook G4 400Mhz back in 2000-ish which came preloaded with Mac OS 9 up to the 2005 iBook G4s which came with Tiger. That's a very big range - post the specifications here, that'll give us a better idea on how much it's worth now.