Re: Can,t find yonkyo
Yonkyo is a control technique first. Look to lock and control uke's elbow, shoulder, and ultimately, center with yonkyo before you look for the spot that hurts.
That said, the kokyu that makes a good sword cut and makes for good kokyu nage also is useful for stimulating that yonkyo spot. Dont just try to push on that spot, try to lift (a standing) uke by that point and let their body weight put the pain on them. Make the spot a fulcrum.