Well, the nerve that I'm guessing is affected in yonkyo as I know it (pressing laterally and somewhat anteriorly on the distal third of the radius) is the superficial branch of the radial nerve. I guess, you could technically injure the nerve if you were doing excessive amounts of yonkyo (and I'm guessing it would have to be a lot
of yonkyo.) However, the superficial branch doesn't feed any muscles, just carries sensory information to the posterior thumb, pointer, and middle fingers. So any damage would simply cause numbness to these areas. And even this would probably go away by taking the stress off of the nerve. Permanent damage? Possible, I guess, but again, I think this is very unlikely. So don't worry about it; there are a lot of worse things that aikido could do to you. Worry about those!
*shrug* But I'm a novice with this stuff. Hopefully someone more knowledgable will step in and take over.
Sarah, finished thinking she might know something about anything