If you really wanted to, you can drill a 4 inch piece of wood on top of your post (assuming it's a wood post) then put the insulator on that for more height. But most fences I've seen are about 4 feet.
I think I will feel more comfortable with a hotwire along the top- I was just always told that 4'6" was the shortest a fence should be.
So multiple people here use 4' fencing with no issue? Perhaps it will be okay then.
I used one strand of electrode plywire - thin electric rope - at 3.5 feet for Casey my 13.2 hand pony and had no problem. I have upped it to 4 feet when I got Candy - 15.2 hands - but am going ti have to do somehing different because she has been getting out.you should have no problem with 4 foot board and one strand of electric.