jason
4" threaded fitting?
perhaps a fire hose fitting might work.
then you could adapt it to a standard "stinky slinky"
after building my pnuematic spud gun there is nothing you can't build out of pvc fittings.
find something that threads onto your trailer then find something that will fit your preferred hose setup. then go to home depot and start shopping.
all you'll need is a can of glue and your imagination!
john