I'll be curious to hear what the OP decides on when all said and done. Because to me "16-18 tt" and "for four", is an oxymoron ! :)
I can't imagine it being acceptable, let alone fun.
Having said that, my funfinder 189FDS has a sideways queen bed with a bunk for one above that, and a dinette that makes into another bed. And I suppose in a pinch, you could put another person on a air mattress or cot on the floor.
However, as is pretty typical of trailers this size ( 19' ), you are not going to get some of or most of the features the OP has outlined. When dealing with such a short trailer, compromises are part of the deal.
In my mind, under 20' trailers are ideal for one person, and possibly acceptable for two people, if they really like each other a lot.
