I think it helps to think of yonkyo as a gripping technique first. The main lesson of yonkyo, to me, is the way in which you use your hand and transmit power from your hand to the target (most likely the lower arm. Once you understand this you can do yonkyo anywhere you can grab, and all your yonkyo's will be powerful.
When looking at the classic yonkyo. It will be easiest to work on the lowest part of the arm possible, and try to find an uke who doesn't have a large forearm in the beginning. When looking for the "yonkyo spot" don't mover around quickly, moving your hand every time you're not on the mark. Instead slowly move around the lower arm until you see a reaction from uke, then work in that area. here's a video that might help: