If you have a flat 5 connector on the truck, you don't need an adapter to a flat 4 trailer. Just plug it in, and the reverse wire doesn't plug into anything. I wired a flat 5 onto my truck, because my boat trailer has surge brakes, and requires it to back up. I've pulled trailers with flat 4's fine.

 

Ill have my trailer back next week and I have to rewire it anyway, so ill make sure its all ok! Thanks!!