There are people doing kits for Honda quads to bring them up to standard, I was quoted £450 - £500 for a dealer to supply but not fit, 2 different kits from 2 dealers. One had trailer electrics, one didn't. Both involved a new front bullbar to carry the lights and indicators, and completely new lighting at the back.
I used to use my 450cc Honda Foreman to launch a fairly heay 18' dory / 85Hp on a rollercoaster trailer without problems but it was on a fairly sound surface. The problem with quads is not power but getting enough grip to use it. I also regulary use it to tow 1/2 tonne of firewood in a trailer across a soft peaty hillside without problems as long as you don't stop on the really soft bits.
Something at least 300cc and agricultural will have no trouble with a 4m rib. There was a nice 2 or 3 year old Honda Foreman 350 that was licenced and 'on the road' on Ebay a while ago, think the reserve was about £2500, also don't think it sold....
A word of warning, they're very 'desireable' to certain people, the Scottish police say that 1 in 3 quads will be stolen at some point in it's life and 50% of private sales are not legit, so check out the chassis numbers with the police before buying. There was a gang operating in the Highlands, just driving around with a horsebox scouting for machines then returning at night.
"I'm not lost, I'm exploring...."