how to boot from USB
You can boot from USB on an iMac G4 using USB 1.1 by connecting the USB drive and following the system's setup instructions.
Check EBay for a used USB 1.1 flash drive, install an OS on it, remove the Mac G4's standard hard drive, and you'll have it done.
You need to remove the HD drive, and it should be an old flash drive.
That's my assumption and the plan I'd take. But are flash drives backward compatible? I'm not sure.
Maybe. I could picture the OS running slowly on version 1.1.