Well do the math. The maximum angle you can ride up comfortably is say 45 degrees. Assuming 45 degrees then solve for x to get the height of the pivot.
(1/2 your board length)^2 = x^2 + x^2 (Pythagorian Theorem)
4^2 = 2x^2
16 = 2x^2
8 = x^2
x = square root of 8, which is like 2.8 ft. or 2 ft 9ish.
So if you go 2.8ft you'll be riding up a 45 degree slope which sounds a bit steep to me still. I'd go 2 feet which gives you an angle around 35 degrees.
Easy way: to find the angle you'll be riding up type in to your calculator 'sin('height of pivot' divided by '1/2 your board length')' to get the angle so in this case it's 'sin(2/4)' which I'm guessing is around 35 degrees but I don't have a calculator with me so I can't say specifically what it is.
