I may have been premature in mentioning the diodes. It initially appeared that BRP had tied the brake and turn signal circuits on each side together, to change their existing six circuits to the necessary four. Something had to be there to prevent back-feed if that was done, hence the diodes, which are like electrical check valves. Based on information posted the last few days, it is possible that BRP has actually changed the trailer wiring module instead, to provide four-wire output to all trailers. If that is the case, the diodes would not be necessary. I have to do some further testing and research to confirm this. What you need is either a matching 9-pin connector on your trailer (using four wires), or an adapter to go from 9-pin to a female 4-pin, which would then match your existing male trailer plug.
Either is readily achievable with the 2011 Trailer Tongue Harness P/N 710002442. This harness has the mating 9-pin plug on one end. and four wires to the trailer. You could cut off the unused plug at the far end of this harness and wire the 9-pin portion directly to your trailer, either cutting off your existing male 4-pole connector, or tapping in to the wires and having dual connectors in parallel on the trailer. You could also do the same with the Deutsch connector parts, in effect building your own equivalent of one end of the BRP harness. I wish I could explain in a way you would understand, but I am out of ideas of how to go about it any better. Please forgive!
Here is a picture of my adapter made from a BRP harness. Mine uses all six wires and a 6-pole connector, for reasons aplicable only to my particular needs, but a four or five wire version would look virtually the same.