First of all, I commend you for acknowledging the importance of going slow with a four-year old hunter prospect. Not only is he still developing physically, but mentally as well. The stifles and hindquarters need to become stronger through hill work (if possible) and transition work. Trotting is the gait to develop strength and conditioning, as well as teaching a horse correct jumping technique.
I really don’t like to jump the four-year olds higher than 3′, and usually not until the fall of their 4th year. If your horse jumps easily and isn’t showing a tight front-end and real use of his back and hind end, it may be due to the fact he has scope and he doesn’t need to make a big effort yet. Often times the horses that are so expressive and stylish over low jumps are also limited in the scope and stride department. I’ve had young horses that didn’t show great style until they were closer to six-years old because they were still growing and finding their balance. So there isn’t a concrete age or fence height that is going to determine if your horse is going to make it in the hunter ring, due to the fact every horse is an individual and matures at different rates. So be patient and don’t get suckered into trappy gridwork or gimmicky bits or draw reins to try to make him jump better. It’s best to teach him basics and let him develop at his own rate. Then you will find out if he is best-suited for the hunter ring or not.
Also love and respect Julie’s take on training and riding. So considerate of the horse!