The Gray installation disks are machine specific. You cannot install a system for a iBook G4 on an iBook G3. However you might be able to clone a system onto it (I'm not sure if this will work but it's worth a try.) Boot the machine up in Target Mode with your G4that the disks were made for, you can try to install directly or try cloning the G4 system onto the G3. Use something like SuperDuper to clone: http://www.shirt-pocket.com/SuperDuper/SuperDuperDescription.html