Yes you need a WD hitch, and since it came with one why not use it? If it's a bit rusty just give it a coat of paint, no need for a new one.
An unloaded weight of 4800lbs is probably going to push 6000lbs once it's loaded which will put your tongue close to, if not over your max of 720lbs. Take a look at your reciever or owners manual too. Most will list 2 weights, one for weight carrying and one for use with a WD hitch which will be higher. I'd suspect that the 720lbs rating is only when used with a WDH.
Also, that amount of weight hanging off the back of a vehicle can significantly reduce the weight on the steer axle and cause control issues. The WD hitch will return most of that weight. You'll want to use some sort of sway control too with the short wheelbase tow vehicle.