Yonkyo is not about preassure points. As any Aikido technique itīs about taking ukes balace. If you however is intrested in inflicting pain it should be noted that you don't need to find a certain point. The preassure is applied on nerves that run near the bones. These nerves are located on every bones in the body. Try doing yonkyo on someones leg.
If you dont "find the spot" i suggest you press harder in order to push the muscles that ar in the way of the nerves aside, or trying to find parts of the bone that isn't covered with muscles.
Jumping in a bit late here...
I've been in dojo that practice rokkyo, and other dojo which do not.
The wakigatame that Chuck mentions is like rokkyo, but the way I've seen it done more frequently is as a ground pin (rather than standing). Often, it's done during tanto dori.
As far as yonkyo goes, my limited experience with the technique has been that I've had to work with it as a balance-breaking technique rather than a pain-compliance one. Sometimes, I just can't get the pressure point. Also, as far as I'm concerned, I would rather train to control than train to cause pain...