Lol, you're braver than me! My horse has jumped 4' competitively, highest I've done is 3'6" and that was one vertical. I think I could work my way up to 4' if I was riding with someone else who jumped that high though... I have this thing where I like to one-up whoever I'm riding with, so if we put the jumps up to warmup height, then raise them to work height, and then put them up one hole at a time, we could end up going crazy high.
But never on my own, and never when I only have to jump 3'4" to one-up the person I'm riding with (that's the only other person within riding distance who jumps!)...