Jo Adell wrote:
Why do Yudansha train with Mudansha? To learn, to teach & to train.Or, because they are there.
this would be my answer, too - in general. That's why I asked for THIS sandan, who after failing in applying a techniques runs to the chief instructor and wines about that ugly orange belt - I do not want to judge, I said only, how it looks like after only hearing one side's aspect.
About kohai-sempai communication, it is fine, if it works at your dojo. I guess there are limits, though. And between 4th kyu and 3rd dan there is a lot difference in experience and hopefully skills.
The other reason, why my sensei introduced that strict rule is, that he doesn't want to have too much chat on the mat, he wants his students practise. It is not really lived that strict. One grade higher or lower is seen by us students as equally graded, and from acceptance point of view, effective skills are more important than formal grade.