I agree, if you and your horse are new to driving, get a 2 wheel cart first. If horse gets scared/spooked, they back-up easier, and are easier to handle. Need to be sure you have a forward driving horse before you get a 4 wheeler, and you should know your horse and how he reacts to things you will encounter on roads and trails. Just believe you should have alittle experience under your belt before moving up to a 4 wheeler, as your horse should also. Also like brakes on a cart, be it 2 or 4 wheeler, really come in handy on inclines.
I started out with a 2 wheeler and once I was comfortable with my experience, and that of my horse, we moved up to a buggy/marathon cross w/5th wheel as I also heard, and have found out for myself that they are safer than most regular 4 wheelers when you drive on alot of different terrain. Also love the turning capabilities of the 5th wheel, mine has a complete cut under (no bar).
My 4 wheeler weighs more than my 2 wheeler, but is easier for me to move as it's balanced on the 4 wheels. Horse likes driving the 4 wheeler better too, no weight on his back.
My horse is 13.3h, weighs about 765 lbs and has NO problems with pulling the 350 lb. 4 wheeler.