I think it's possible the 5" deep trench guy may be 5' average depth meaning a course at 6, a course at 5 and a course of loop at 4'. this would not be uncommon in MI.

I want to reiterate a previous comment of mine. There is no way a HUD minimum (trailer) duct can support a 4 ton geo.
Loop design doesn't matter if this is not addressed.

I also suspect 4 tons may be bigger than project requires. Though trailers can have a heavy load, what is the total SF 1,800? A 3 ton may be adequate and may fit ducts. We need a man. J load calc.
Joe