Realize there is a huge cutoff point at around 3 feet.
Most juniors and amateurs compete quite happily at the two foot nine and 3 foot mark. I’m watching FB posts from a local barn that has gone 1000 miles down to Thermal California for the winter circuit at great expense and is posting videos of clients competing at 2 foot 9.
Our rated shows here at home tend to top out at 3 feet but might have a few 3 foot 3 classes that don’t fill much.
Once you get well over 3 feet you are in a different league, at least around here. You go to different shows and you ride different horses. Most horses can do up to 3 feet. But a full 4 foot course takes a lot more ability in the horse. And you probably move to a different training program.
It’s true that some gifted young multi athletes, especially fearless young men, can sometimes move up to big jumps faster than the norm. Especially if they are already really good at things like skiing.
But for most people it is going to take years, and indeed most people don’t compete at 4 feet.
If you want this you are going to need to ride every day, preferably multiple horses. You are going to need to do targeted fitness off the horse. You are going to need a great seat no stirrups no reins. You are going to need to develop a real feel for the distance and stride. Obviously you can and must develop all this at the lower levels. You also need to ride a competent course
When you are competing and placing well enough at the 3 foot level is when you might want to consider if you want to move up to the next level.
Obviously in order to do that you need to own or lease a scopey horse and they are not cheap.
Realize that few lesson horses are allowed to jump above 2 feet or 2 feet 6 to save wear and tear. So even if you want to compete at the 3 foot level you will need your own horse