The pain of yonkyo is usually caused by the compression of periostium, the nerve-rich membrane surrounding the bones. Yonkyo twists the two arm (or leg) bones until their periostia are compressed together causing major ouchness.
There are people who, for whatever reason, don't respond to this kind of pain, which is why you line up yonkyo in such a way as to be able to move their skeletal system.
It is not a pressure point. The pain shows up wherever the periostia (not sure of the latin plural here) hit each other, which is slightly different in each person, according to their bone length and muscle size.
Greg Owens (sp?) did a series of articles on the physiological basis of Ikkyo through Yonkyo.
yonkyo is painful because pressure is applied to a nerve overlying a bone in the forearm. there are lots of pressure points in the body, but in my beginner's opinion, they all are 'a little something extra' and should not be a focus point of your technique---based instead on the MD behind my name and the fact that i'm one of those folks instructors come to with an evil grin when my nage complains i lack the pressure point. Not everyone's body has read the textbooks, i tell my patients. Those who don't feel the pain that i see in the faces of my colleagues may have the nerve buried, or in a deeper place, or even an entirely new location, or they may have a high pain threshold (their body for a variety of reasons doesn't recognize pain in general as most do), or be on drugs, or last, and most dangerous, may be willing to suck up a lot of pain to accomplish their goal.
in th opinion of someone who can stand there literally for minutes with my hand poised to punch a nage intent on inflicting pain, go for the balance instead.