Yes, but only some of the very last models will be able to boot off USB. Pretty much all PPC Macs could boot off Firewire, but only a very select few ever got the ability to boot off USB.
Can G4 Powerbooks boot off the Firewire port?
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Can G4 Powerbooks boot off the Firewire port?
Since we're nitpicking... Some of the very last PPC models COULD boot from USB drives. IIRC, it involved some open firmware magic, so it wasn't exactly a simple matter to accomplish like with FW drives, but it COULD be done.
Now, whether or not the PB G4 the OP has is one of those capable of this is unknown. Generally the last, maybe second to last if the model was refreshed pretty rapidly, refresh of each model could do this. Right before the switch to the x86 platform.