Maybe I can assist - I was a D1 defensive and now my son plays defense for a competitive club team.
You play defense with you feet. A long pole at this age will discourage them for playing defense properly because they are going to be chasing with their stick.
Also, I don't think they 4th graders (especially D) have developed their strong hand/weak hand. So, let them develop with a short stick.
If you son can scope, cradle, switch hands effectively, pass, catch-- and plays defense whis feet - then go long pole