When I first went to install my 7-pin Bargman connector, I already had a Flat-4 on my truck. Hating to cut off a good connector, what I did instead was wire the
lights and ground out of the Bargman to a trailer-end Flat-4 plug and "semi-permanently" connected the two Flat-4s together (gotta clean that sucker and put the grease on it).
Now when I have occasion to tow a trailer with Flat-4, all I have to do is open it up and connect. Handy place to troubleshoot
light problems, also.
Pete and Rats