Do you have an external transmission cooler installed? It is an aboslute MUST if you're towing any decent weight, which you are.
See recent discussion here:
https://www.piloteers.org/threads/bigger-transmission-cooler.186647/?post_id=1868608#post-1868608
Also, if you got the transmission oveheat warning, your ATF is absolutely toast. You need to do a 3x drain and fill immediately to get as much of the old burnt fluid out of there as possible. I recommend you (1) install a transmisison cooler, (2) switch to a good synthetic ATF that handles high heat loads better than DW-1, (3) also recommend you change out your transfer case and rear diff fluid.
As for using D4, I would
if the transmission is hunting and not holding gears well. Speed is a killer with a trailer; you can't go 70-80mph with it...it just puts too much stress on the engine, transmission, and cooling system. Slow down (as per the instructiosn in the owners manual) to 55 or 60, and let yourself slow down when climing hills. This is the reason people like pickups and V-8s for towing; they have the power and spare capacity to power up hills; the Pilot does not.
