I've jumped about 4'3", but that was a while ago and it was only a few times. I was working an Oldenburg gelding and he really was a nutter at smaller fences, just because he didn't respect them. After the jumps topped 3'3" he'd settle down really nicely and behave himself (mostly). One of our regular exercises was trotting a 3'6" vertical. It really made him think about how he was jumping and where he was putting his legs.
That's a video screenshot of us trotting 3'6". It doesn't look that big...mostly because Donald is like 17.3 hh, lol.