4 isn't too young to start poles, but I wouldn't get on her back any earlier than 3. Also, you should get your vet out there to make sure her knees have closed. I have a five year old that I've just started doing larger jumps with. I started him over ground poles at 3.5, cross rails for a year after he turned four, and this summer I've started doing boxes, trees, fences, tires, etc... But it's not really a specific thing, go at the speed of your horse: you'll know if he has developed mentally enough and your vet can tell you if she is physically able.