You say you reformatted it with and then restored from your (Intel) MacBook Pro to use in your (PowerPC) PowerBook G4. Intel-based Macs can only install and use Mac OS X on a disk with the GUID Partition Scheme, while PowerPC-based Macs can only install and use Mac OS X on a disk with the Apple Partition Scheme. Your MacBook Pro would have been using the GUID Partition Scheme which PowerPC Macs can't boot Mac OS X from.
Follow the instructions in this article:
to reformat the drive you will be using in your PowerBook once again, making sure to repartition the disk after reformatting (otherwise, the problem won't be solved). Of course, instead of choosing GUID Partition Scheme as the article instructs, choose Apple Partition Scheme (since this article was written for the opposite situation, putting Mac OS X on an HDD in an Intel-based Mac partitioned with the Apple Partition Scheme, which wouldn't work in Intel-based Macs).
Once finished with this, if you still want to use your Mac OS X install from your Intel Mac on your PowerPC Mac, then cloning your Leopard installation from your MacBook Pro disk to your PowerBook G4 disk (using SuperDuper or Carbon Copy Cloner) should work.
Note: Your PowerBook G4 supports up to Mac OS X v10.5 Leopard while your MacBook Pro supports up to Mac OS X v10.6 Snow Leopard. Make sure you aren't trying to put Snow Leopard on your PowerBook G4, it won't work!