Horse starting isn't as hard as some might think, but it does require a good bit of time and patience.
From personal experience, the younger the horse is, the easier it is to start them (waiting until they are four years old is a BAD idea.
 Once a horse has been doing it's own little jig for a long time, they kinda get set in their ways.
Horse and Humans are alike in a few different was one being, it is pretty easy to teach a child new habits the same is true with baby horses.
A four year old horse is equivalent to a fifteen or sixteen year old child.
Say a sixteen year old child has gone their whole life so far with few rules then all of the sudden their parents want to start enforcing new rules, chores, and habits, you know what would happen? sparks would fly that would fight those new rules with every thing they had in them.
The same thing goes with a horse if you wait till they are four years old to try and start them it probably wont go over well.
while it is possible their is a lot of unnecessary risk and difficulty involved).
One could begin the horse starting (or some call it horse breaking) process when the foal is as young as 5 hours old!! You may say that's ridiculous, but the first step with starting or breaking a horse is developing a trust or a bond with the horse, You could do this by petting, loving, brushing, grooming them etc.
, after you gain their trust anything you want to teach them will be much easier.
Now I'm definitely not saying that the process will be easy, whether it is or not all depends on the horse and their personality (or Horsonality).