Starlight. ime 4 is plenty old enough to take individual piano lessons with any reasonable teacher, perhaps start with 30 mins. At that age (and some more years later) most good teachers will expect you to sit in and possibly take notes too.
His musicality will pull him along, finger etc size are not a barrier. It's hard to generalize, but I chose not to teach my children myself, because at 4 they were old enough to know the difference between business and personal, but too young to separate the roles in one person.
Unless you're committing to Suzuki or a similar approach, some find group lessons (rather than group music making) musically ineffective. It may be the English way, but seldom found elsewhere in the world.
I'm not convinced by music therapy either. We can all put together a decent sound, but to repeat and vary and develop those skills is another matter. A good teacher will always be part therapist, will work with the distractions, the physical barriers, the communication issues. I hope you find the right one soon.