Q: Need to retrieve or erase HD on Power MAC G4 (2002) and Mac IBook (2005) and then dispose of both of them.  Neither will boot up.

Without the ability to boot, about the only path left to you is to physically remove the hard drives and send the computers to recyling without drives. YOu can retain the drives fror reuse or destroy them (beat with hammer; drill holes through it. etc).

Remove drives:

1) PowerMac G4: https://www.ifixit.com/Guide/Repairing+Power+Mac+G4+Quicksilver+Hard+Drive/2079

2) iBook G4 14-inch: https://www.ifixit.com/Guide/iBook+G4+14-Inch+1.42+GHz+Hard+Drive+Replacement/73 1

     iBok G4 12-inch: https://www.ifixit.com/Guide/iBook+G4+12-Inch+1.33+GHz+Hard+Drive+Replacement/70 3

I later realized that the iFixit instructions I linked for removing the drive from an iBook may seem a bit too "fiddly" and they are. Understand the instructions are for replacing the drive so cautions are there to keep the computer working after the replacement. As you are going to recycle it anyway, you don't have to be so cautious.

Thanks for the "Solved."