Assuming the G4 PowerBook is running MacOS X ...
01. Select the 'Apple, About This Mac' menu item. The 'About This Mac' window will appear.
02. Click on the 'More Info...' button. 'System Profiler' will launch, displaying a window with the left side 'Contents' column's 'Hardware' selected. In the right side, displayed should be 'Hardware Overview:'. The last item 'Sales Order Number:' is the PowerBook's model number.