Just my personal opinion, but I believe the "nerve pinch" has nothing to do with a good yonkyo.
Ikkyo, nikyo, sankyo, yonkyo, gokyo. They are all just different ways of breaking uke's posture by connecting (a.k.a. taking the slack out of) through his/her arm.
A good yonkyo is, IMHO, difficult because it is difficult for nage to exert energy through that particular connection.