Welcome to this great forum Pedro!
The wire that runs the low beam on your H4 headlamp wi;; bee fine to use, Than't the black wire with the green tracer.
You should also be able to use one of the wires from the directional harness on the left or right side of the bike. The color code for those wires is blue on one side of the bike and blue with a red tracer on the other side. Then you will provide the ground.