There's no reason that won't work on a temporary basis, but I wouldn't waste any money on an adapter. If the trailer is coming to you with a seven round, it likely already has brakes, and probably a charging line, too. If I was in your position I'd change the tow vehicle connection to a seven round, or add one next to the four flat. That way you have all the flexibility you need for adding brakes/charging line back in.
On that subject:
Dunno about your State, but in mine, if a trailer is already equipped with brakes one is required to use them whether or not the trailer's "under the required weight".