I also wonder if it might be time to look into EPGY for math. (Would go through an open enrollment group so you don't have to get testing and because I hear it is much much cheaper.). Honestly if you have a 4 year old happily doing double digit addition, I doubt holding him back is the way to go. You might be postponing a battle best fought now. He might be happier & by the time he is in regular school, you will have documentation from Stanford of where he is and why regular unaccelerated math won't cut it.

Just a thought. Fwiw my kiddo is way younger and not tested.