I have a Deere 3R series with 320R loader and a 16 ft Big Tex 70TV 7000 lb. trailer on a 1/2 ton pickup with an 11,000 lb. tow rating. Loading that tractor onto that trailer is a bit of a challenge for the tow vehicle, as you need to play some games to get the weight distribution just right... 1/2 ton trucks don't respond well to massive tongue weight. Based on that, I'd opt for a longer trailer even for my 3R, and scale up from there for a series-4... maybe 20 feet.
Also, with just a 16 foot trailer, carrying implements with the loader mounted is always a challenge. I have to play games with backing onto the trailer to drop an implement up front, then turn around and put the loader bucket on top of the implement, or other such stupid stuff, trying to get everything fit onto the trailer while also nailing the tongue weight close enough to that 10% - 15% window, for the trailer to ride level and not sway.
Whatever trailer you end up with, please definitely consider a WDH for the Ranger. That's an awful light truck for pulling a series-4 tractor, no matter what trailer you choose.