Breaking in a 4-year-old:
For the first sessions, before assessing the horse's temperament, prefer a simple bit, or a snaffle bit, with thick double-jointed cannons; if this proves insufficient for control, choose thinner cannons.
The horse does not stretch into the reins:
Some horses are afraid of the hand (their past is not always known), in which case prefer a straight resin bit, a soft or hard rubber bit with or without a curb chain so that it leans into it with confidence.
When you feel a real contact or too much contact, switch back to a more or less thin double-jointed bit or one with a curb chain, depending on the horse's temperament.
The horse has a very delicate mouth:
There are more uncomfortable bits than delicate mouths.
Try a double-jointed bit of the correct width and adapt the cannon diameter to the horse's temperament.
A straight soft or hard bit can be the right choice. A simple bit with a very thick cannon is only comfortable for the corners of the mouth, but not for the palate.
Remember to consult the dentist or osteopath.
The horse, head down, tries to pull the reins out of your hands.
This is a signal that can mean: you are hurting me or you have too much hand.
Reread the previous paragraph.
The horse "drops the hand":
This sometimes happens in the last strides before approaching an obstacle; try a less strong bit, for example, a pelham with short shanks instead of one with long shanks.
The horse pulls a lot:
Before entering the "always more severe" spiral, proceed gradually by playing with the size of the shanks or the thickness of the cannons.
Simple or twisted double-mouth bits, and a thin gag bit are good choices.
There are also hackamores with long shanks that can be coupled with a thin curb bit, even a very thin small gag bit.
Above all, you must try the"Alain Ventre" jumping or pelham bits, which with little hand give a lot of control and relaxation because they act little on the mouth. The effect is transferred to the nose and the chin groove. After a few sessions, the work is done calmly, the horse progresses, and balance follows very quickly.
In all cases, remember that the parameters are different if your horse is ridden by a professional or an amateur and it seems fair that the same bit may not suit both. In all cases, it is the opinion of the coach you have chosen and who you trust that is authoritative; they are the one who knows best the partnership you form.