If that's what you'd call the sort of vertical, but mostly lateral launch they do when striking. My point was more that they can only extend themselves so far

If that's what you'd call the sort of vertical, but mostly lateral launch they do when striking. My point was more that they can only extend themselves so far up a vertical object before they just fall over. A solid 4-foot fence is plenty as far as most snakes trying to get over it. A 6-foot fence will defeat any snake headed in the upwards direction. Your biggest concern, should you be trying to exclude snakes, is that they burrow and it doesn't matter how high you build that fence if you don't sink it into the ground a couple few feet.