Etrailer,among other places, sells a 7way connector that has a flat four pigtail as one of the inputs. That way you just have to work with the new wires for the brakes, 12v power, and backup lights and you don't have to mess with the four wires you already have.
Search the forum for comments from people who have actually towed a TrailManor with an FJCruiser. I don't recall any members reporting any issues because of the wheelbase. A Weight Distributing Hitch should be considered mandatory for anybody with a wheelbase under 150", in my opinion. It's probably a good idea there, too, because it keeps the trailer from unloading the front wheels, which are real handy for steering and braking.