Most times the USB thumb drive does not make a suitable boot volume for G4 Macintosh in OS X. If an OS X is installed or clones to a USB flash drive, the formatting has to be correct and sufficient free space must exist. Plus a few other rules that are widely published need be applied. With some drives, they need to be self-powered and not use a port on the subject computer; the power is not there all the time during the boot cycle, so the external drive using only port power may not be seen at all. This may vary, but especially when troubleshooting or booting from a system clone, etc.
•Mac startup from open firmware to OS X - search result link:
https://www.google.com/#q=Mac+startup+from+open+firmware+to+OS+X
In the later G5 iMac, there was a change in firmware to allow USB flash drives to be used as boot volumes. Some unix variants do OK from a flash usb drive, and the OS can be tweeked in Terminal. Then there are Developer pages...
There are advanced user databases at Apple Support involving the use of such behind-the-scenes technologies and developer discussions, perhaps you should register to use them? It does not cost.
There are times when it may be possible, but when they will work, they should not require you to run the computer in Open Firmware. And some of the steps with associated questions lend questions of purpose that also may breech the terms of use and other intellectual property rights when copies are made, etc.
So, that may be why some questions across the discussions threads could go unanswered. Some are due to lack of an appropriate reply, or unwillingness to commit to some kind of reply that may also be marginal. And as these machines, OS X versions & user base fades, fewer would be able to formulate a reasonable reply.
Hopefully you realize there were a few who did read the question.
Some may have not understood it as a day-to-day user inquiry,
and others probably did not want to research an answer online.
Sorry to have said nothing until now, & even now.
Good luck & happy computing! 🙂
