Try going to system prefrences and then share on the iBook. Select share folders and files.
Using your MacBook Air, go to network from utilities on the top menu and then network.
With your iBook G4 powered on, connect to the iBook which should now appear under network (Make sure they are connected to the same network or wireless)
Connect as your iBook user name and password, navigate to the desired folders or files and copy the content to a location on your MacBook Air.
Best of luck