Is this a quicksilver or graphite G4? I know it's possible on a quicksilver, this is all you should need to do:
1. Mod your CD drive and make the tray door as narrow (snip off the extra shit on the ends, and pull off the face plate) as possible
2. With an allen wrench, pull off the front handles/feet on the G4, now you should be able to remove the front panel with a little careful prying at the right spots.
3. Once that's done, remove the zip carrier inside the computer, you wont' need that, and remove the metal shim covering up the zip bay partially on the outside shell of the G4 (metal case)
4. Once that's done, grab some duct tape or other mounting tools, and attach your DVD drive to the bottom of the CD-RW or whatever is the main drive, with spacers or something so it lines up with the opening of the case.
5. Reassemble case, and keep off the zip drive cover, if you want, try some sort of spacer between the zip bay cover (plastic thingy), and the drive, maybe foam, etc, and snap off the clips on the zip bay. You may also just leave it alone without any face cover.
Hook up everything, and enjoy your two drives
