Quick ? about the H4 headlight upgrade - ground circuit.

The ground should be the same gauge or larger than the supply wires. So if you build a headlight harness with 14 AWG wires all three wires for each lamp should be that size. The ground should be as short as possible connected directly to the body.

Keep the factory wiring as-is for the markers and make you supplemental headlight harness. Pigtail the ground to a headlight bucket screw.