If the computer could still start in Target Disk Mode, and be seen as an external hard drive
by another Apple computer that could connect back to the old G4 by correct cabling, to the
FireWire port on the iBook, then you could see if that old semi-failed hard drive could be
erased by using Disk Utility in the working computer.
To be able to erase the content should be adequate if the drive can be seen by another Mac.
The computer may be still usable to someone who likes the older models, and if the only thing
not working is the original hard disk drive, someone could want the computer to fix & use it.
Someone may be willing to remove the hard drive for you, in trade for the computer. Or for
a technician to do so, would cost. It may be worth considering while the iBook is open, to
have a known-good replacement hard drive installed. The labor is near similar, once inside.
For a bare-drive to connect to a working computer, so as to try & erase or reformat (or access)
a common item is a wire kit something along the lines of this; other companies offer a like kit:
• Universal drive adapter USB kit: http://eshop.macsales.com/item/NewerTech/U3NVSPATA/
Fairly much the same info on how to prepare for sale, or give away, or donate will apply;
except for the part where the original internal drive doesn't function. Here's another sunny
day method for disposal http://www.thesafemac.com/how-to-prepare-your-mac-for-sale/
Sometimes an older computer may have some life left in it; however the older hard drive ATA
IDE (Parallel ATA, not SATA) may be more difficult to find. An authorized Apple service provider
may have some known-good used pulls, but for the effort, if resorting is intended, replace a
bad drive with new. If the vintage obsolete computer does not have any other issues pending.
Once the old hard drive is removed, in any case, you could choose specific other methods to
make the information on it inaccessible by most conventional means. A big hammer, maybe?
In any event...
Good luck & happy computing! 🙂