You have not said which PowerMac you are using. The lower front bay was intended for a Zip drive on most PowerMac G4s, it can also take a hard drive, but not an optical drive. The MirrorDriveDoor models can take a 2nd optical drive in the lower bay.
Replacing the optical drive isn't too hard. If you want maximum compatibility, get a Pioneer 108 or 109. Both have been used by Apple as Superdrives, so OSX has good support for them. Some other drives will work, others will not - or will work, but not boot the system.