The Power Mac G4 can boot from a FireWire storage device that implements SBP-2 (Serial Bus Protocol) with the RBC (reduced block commands) command set. Detailed information is available from Developer Technical Support at dts@apple.com.
It is possible to connect two computers to each other using a FireWire cable, but no software is provided to make use of such a connection.
For additional information about the FireWire interface and the Apple APIs for FireWire device control, see the references shown in FireWire Interface.