just my opinion and backed up by the wiring diagram in the triumph manual
the 4 prong relay is a normally open relay
the 5 prong relay is just like the 4 prong except that it has an extra prong that put power to the lights
when not energized.
when u hit the starter button the power the relay is energized and breaks the light circuit and applies the starter solenoid circuit.
again just my opinion
in an emergency u could use a 4 prong relay to replace the 5 prong starter relay but your lights would not work.