Technically, the PowerBook’s G4 don’t have the needed firmware to boot from USB drives.
I only know of G5 based systems having a tweak within its release of Open Firmware that could do it, but there are a few issues just the same! You need a USB 2.0 drive and the file system needs to be the older HFS not the HFS+ to work.
If you want to pursue this I would recommend you locate the grey disks that came with the system or OS-X Leopard (10.5.8) retail disk set. Then you can boot up under the CD/DVD that come in the set (depending on what optical drive your system has). Your other option is finding a FireWire drive that is setup with a bootable OS-X Leopard on it.