First, you must back up all the contents of the disk. Apple says the OS X partition should be the first partition on the drive. Next, restore your data and your OS 9.0.4 System Folder to the 4GB partition. Finally, install OS X on the 2GB partition. A more palatable scheme might be to add a second hard disk - either internal or external.
External drives cost a bit more.
You'll need to check your user manual to determine whether you should install an internal ATA or SCSI drive. But there are some limitations. If you have a G4 Mac equipped with an ATA hard disk and you install a second ATA hard disk, you must install OS X on the original hard disk. If you have a Bronze Keyboard PowerBook, you cannot install OS X on an external SCSI hard disk. Also, you cannot install the public beta of OS X on an USB or FireWire hard disk.