sorry the truck was left out of the picture on this excursion. However, my friend is no stranger to pulling big things with his pick-up. Here is a pick of his truck last summer.......
70ft long, 14 ft wide mobilehome being towed 3 miles......the first and last time he will do this
From a past life, many years ago while I was a forklift operator at a warehouse... I Have some information for your friend.
One day, I decided to go out back and lift the tongue on a construction trailer (empty), same size as you got there. Looked just like it but it had some construction company's name on it. The tongue was sitting on a stack of pallets and the pallets were showing some fatigue in areas. My plan was to lift the tongue and slide some new pallets under it. The ol Toyota wouldn't do it. The hydraulics bypassed on me. Never had that happen before. At the time, I knew I could lift two 40"x48" pallets of pizza sauce in cases of six #10 tins, 40 cases to the pallet. That's +/-4000#. The rearend of the jitney would get a little light, but if you took it slow it would do ok.
Trying to lift that trailer, all I can say is the tongue weight exceeded 4000#.
If your friend is hooking 4000# of hitch weight to his Dodge, that's great but he's going to break something. I pray not on a public road.
4000# is doable on a gooseneck, but on a receiver hitch it's too far behind the rear axle. Alot of leverage there!
Just because the truck can pull it doesn't mean the truck can tow it.
He can take that for what it's worth to him.