400 lbs. of tongue weight pulling a 4500 lbs. trailer will cause you a lot of problems and sway will definitely be one of them. You need at least 550 lbs.....600 is even better for control of the trailer. Can the Atlas take it?Owner here of a 2018 SEL-P towing a 4500 lb travel trailer with a 400 lb tongue weight.
The trailer came with a WDH which I have used to see what difference it provides over a standard hitch. In my limited experience (about 20 hours on the road), I have been very happy with the Atlas towing performance both with and without the WDH. This was all in the relative flat of southeast and mid Atlantic areas, so nothing too challenging.
I will discontinue use of the WDH because of the VW prohibition (although like everyone else I question this).
The friction type sway control bars seem to get good reviews and are very cheap. Harbor Freight actually has one for 30 bucks I think. I may try one.
I also was intrigued by the electronic systems that use the trailer's brakes to reduce sway. Those seem to get reviews also. But are more expensive at 400 to 500 bucks.
I bought the Firestone air bags in order to take out the slight rear end sag I have without the WDH. Haven't installed them yet.