As best as I can understand is that the pain is an optional bonus, but the leverage is the key and they should go down regardless of whether it hurts or not. There are always a few freaks* who never seem to feel pain no matter who does yonkyo on them. But they still get their faces driven into the mat if someone knows how to do it well.
* I mean 'freak' in the nicest, friendliest, least insulting way possible
I agree. To put yonkyo on, you get off the line and then use the forearm to put uke's elbow through their face. The hyperextension of the shoulder that this creates is what puts them on the ground.
In order to get the pain as well, grip uke's wrist with your thumb and your little finger only. The little finger should be in the crook of the wrist. Extend your index finger, and don't squeeze. Then just do the movement described above. The technique will naturally happen.
Having said all that, yonkyo is by far my least favorite technique. I find the pain response to be unreliable, and even though the rest of it is a valid technique, given the choice, I would do either ikkyo or sankyo instead.