Although you could create a bootable backup clone of the current system, that would be
OK for your own uses; and it also would include your personal account information. Since
there is a spotty record of late model G4 iBook computers (sometimes) not booting from
USB drives, a clone on USB flash or thumb-drive; that has a spotty history of sometimes
working as a method to get around the lack of an optical drive.
A second PPC Mac w/ FireWire + optical drive, may be used as optical drive to help install
an OS X from the original media install-restore DVDs. The Target Disk Mode may work, if
you have a second computer that has FW ports.
• How to use and troubleshoot FireWire target disk mode - Apple Support
There is a way to erase personal information by use of terminal or single user mode, but
I've not done that; a command line could allow the computer to start up without your old
account, and set up a new user account. However the lack of system install-restore DVD
leaves any later user without the correct means of fully re-installing a system, or replace
the hard drive; since Disk utility is on the disc and is a primary tool for recovery.
An externally enclosed (with an external power supply) FireWire optical drive with a chip
set that recognizes the ability to boot an OS X system from that drive, would be ideal. Or
to get a replacement optical drive (and hard drive, while its open to repair) and fix it.
I have several items like that. Including clones of PPC systems on FW external HDDs.
So much fun, only to a point.
Good luck & happy computing! 🙂