My '83 C22 had a 2-conductor setup forward of the maststep for the steaming light, which I changed to a 3-conductor setup. I could have used a 4-conductor, as you propose, but I chose to use the same ground wire for the two lights.
Because I added the decklight to the steaming light, I had to remove the 2-conductor connector at the deck, drill thru the deck and the headliner inside the cabin and run new wire for the lights back to the panel and battery. I bedded the connectors above and below decks with 3M 5200 and ran the wires behind the teak trim on the port side of the cabin. I did this job in 1998 and haven't had a problem electrically or with leaks ever since.
Does this help?
Sailing isn't a matter of life and death. It's much more important than that!