I agree Jun! I have really noticed the focus on pain compliance in Yonkyo. More than once I've seen folks standing there in intense concentration trying to find "it". Meanwhile, Uke is standing there looking at them with plenty of time to do a reversal or worse! Quite a while ago I learned a lot from a woman in the mid Kyu ranks that was having nothing but trouble getting some of her thicker forearmed partners to twitch in pain from the technique. She started focusing on not trying to get the pressure point (she just placed her hand on the forearm) and focused on taking Ukes' balance. What a revelation! Once she got that out of the way and under control, she is able to get the pressure point easier and under safer circumstances (while Uke is already off balance or on the ground).
I also understand where Cas is coming from though; it seems the thicker the arm the more difficult is to find the "point of pain". I have found that the thicker the forearm the closer one needs be towards the wrist to get that Yonkyo jolt.
One other point of interest to me regarding Yonkyo is that in Acupuncture, the Yonkyo pressure point is the heart meridian. I took some Tai Chi from a Sifu that also ran an Acupuncture school and when he showed the point where they put the needle into someone having a heart attack, it was right there! He finds the point by putting the web of his hand (between his thumb and forefinger) to the web of the patients' hand (same place) and touches the forearm with his middle finger. That is where the needle is placed. Kinda cool, eh?