If you need to boot an iBook g4 from a memory stick, the official line is it cannot be done. Last week though, I had to do exactly that.
(The CD drive had stopped working and we didn't have a boot CD. We did however have an external bootable drive clone which was made with Carbon Copy Cloner)
1. Plug in the USB drive containing your bootable system
2. Turn on the iBook and immediately hold the keys Command Option OF. It should be in OpenFirmware mode now.
3. Let go of the key combination.
4. Type "boot ud:,\\:tbxi" and it will start booting from your USB device. If it doesn't, go to step 5.
5. Type "dev usb0" and press return.
6. Type "ls". If you see "/disk@1" mentioned, carry on, otherwise go to step 5 and use "usb1" instead of usb0
7. Type "dev disk@1" and press return, then "pwd" and press return again. You should get something that looks like "/pci@f2000000/usb1b,1/disk@1". This info is now used in the next step.
8. Type "boot /pci@f2000000/usb1b,1/disk@1:,\\:tbxi". This is the device name shown in step 7 after typing "pwd" with ":,\\:tbxi" added to the end.
9. Press return. It should now boot. It will be slow, but will allow you to reinstall the system.