Yonkyō
四教

Nerve pain

Traditionally, yonkyō functioned through the pain exerted by pressure on the nervus meridianus. The upper figure shows the inside of the arm and the course of the two nerves (yellow) supplying the hand. The optimal pressure point is shown at the bottom left. The nerve runs here across the bone relatively close to the surface of the forearm. The bottom right shows the edge of the index finger pressing the nerve against the bone.
During training it can happen that Uke's nerve is so maltreated that it remains very sensitive to pain. Then Yonkyō works the whole time without much effort on the part of Nage. However, some uke develop indolence so that yonkyō no longer works through pain. Then uke must be lead by controlling the forearm.
In Ki Aikido Nage does not work with pain and controls the forearm to lead the shoulder blade.

Yonkyō is extremely rare in the Ki Aikido examination program. There is katadori yonkyō in the 2nd kyu program and katatedori tenshin yonkyō in the 1st kyu program. The examples shown below stem from "All of Aikido" (approx. 2000) and from seminars in Bosco Gurin (2015) and Heidelberg (2017).
The last video shows Yonkyō as being practiced in the Aikikai.

Yoshigasaki

Katadori yonkyō irimi

Yoshigasaki

If sankyō is not possible because uke keeps his fist closed, yonkyō is an option. To avoid that Uke cannot puts his arm behind his back - and render in this way the technique ineffective - there must always be enough distance between Uke's arm and his upper body.

Yoshigasaki

Uke hardly bends forward with yonkyō; his back muscles can prevent that. Uke is therefore led away to the side. Uke does not walk around Nage. Nage leads uke.

Michel Erb (Aikikai)

katadori menuchi yonkyō omote
shomenuchi yonkyō ura
In this case omote corresponds to irimi, ura corresponds to tenshin.