After reading the article about the ikkyo pain mechanism, I got interested in checking if anyone had done research into the other Aikido teachings. This one about yonkyo was very rich in explanation (using a cadaver, again) as well as philosophy from leading Aikido shihans about how they think yonkyo works. The upshot is that Saito, Ueshiba and Inoue agree that the motion and angle of the arm under yonkyo is more important than the pain-causing effect. The article authors concur that pain can be caused by pressing firmly on any of the nerves shown in the following illustrations.

From An Examination of Aikido’s Fourth Teaching: An Anatomical Study of the Tissues of the Forearm, Perceptual and Motor Skills, 1990, 71, 1059-1066

See figures 2 and 3 below from the article.

Yonkyo Ura Nerves

Yonkyo Omote Nerves