If you can make an IDE cable fit, you can run an optical drive as the slave if you like. Or the master. Old G4s can be a bit fussy about bootability and jumper settings. Some will not boot from slave optical drives. Others may not boot from any slaves. You will have to do some testing, but you should be able to get a bootable HD and readable optical in some configuration at the very least. You might consider moving the HD to the zip bay to make it easier.