I just found this very informative discussion. In following the link below, I found that it is currently dead (which is not surprising given that it is now 13 years after the posting). However, I've found the full reference for the article that I think it was citing:
Olson, G. D., & Seitz, F. C. (1990). An examination of Aikido's fourth teaching: an anatomical study of the tissues of the forearm. Perceptual and motor skills, 71(3 suppl), 1059-1066.
You can download this on the web from the publisher for free (at least I was able to).
It is very enlightening as to what causes the pain of Yonkyo omote and ura, which are importantly different.
For omote, there are two versions, one of which puts the pressure on the wrist (palm side) a few inches from the palm, on the thumb side from the midline of the wrist. The second version puts the pressure a bit closer to the radial bone, which is further towards the thumb side.
For ura, the pressure is put very much on the radial bone at the extreme thumb side of the arm. This area has little flesh and muscle to protect the nerves beneath.
The article above does not dwell on proper placement of nage's/tori's hands, or overall body positioning of nage and uke, but instead focuses on the anatomical locus of the pain.
For an excellent coverage of the proper technique, including close-up details of nage's/tori's hand placement relative to uke's wrist, see pages 154 and 156 of Best Aikido: The Fundamentals
, by the 2nd and 3rd Doshu, and translated by John Stevens:
Ueshiba, K., Ueshiba, M., & Stevens, J. (2002). Best aikido: the fundamentals. Kodansha International.
Getting back to a theme throughout all of this discussion, however, Olson and Seitz (1990) note several reasons why using Yonkyo is probably impractical for most people in real-world situations, as noted in a number of the postings here (e.g., the placement of nage's/tori's hands requires precision, which cannot be learned quickly, attacks happen very quickly, and some people's pain threshold is so high as to make the pain of no consequence). So, securing uke's shoulder to the ground is of primary importance, rather than hitting the nerve spots and causing pain.
Nevertheless, for those practitioners wanting to get better at hitting those nerve spots, the above references should be very helpful.
Tatsumaki Aikikai/KSU Aikido