Personally, I would wait until she is 5 or 5 1/2. She just won't be developed to handle the stresses of jumping, in my opinion. I think in todays horse world, people start horses in competition and training waaaaay too early, and I hate seeing 2 and 3 year old horses in reining and cutting competitions, for example. It makes me sick. Also, at 4 her pelvis may not be fused yet, and if it's not, she could develop a jumper's bump.
Ground poles are great, raised cavelettis are great, but I would be leary of jumping her at 4. But that's just my opinion.