Some years ago I read in Anthony Glise’s seminal book, Classic Guitar Pedagogy, about Sor’s 4th finger approach (this was adopted by Aguado in the following decade). After I examined Sor’s method (early 1830’s, I believe), I found that Sor clearly showed that the “D” on the 2nd string and the “G” on the first string should be played with the 4th finger, not the third. Since Sor’s guitar was most likely a smaller 630mm scale Lacote, and as he was one of the greatest guitarists who has ever lived, I doubt that he had difficulty reaching the 3rd fret with the 3rd finger. It seems to me that the only reasonable explanation for the 4th finger on the 3rd fret is to reduce the amount of pronation of the left-hand.
I tried this with a few students, young and old, and the problems with pronation were significantly reduced. Their hands stayed closer to the guitar neck and “G”, “D”, and “Bb” on the 1st, 2nd, and 3rd strings, respectively, became simple. Finger exchange difficulties were more easily dealt with. I continued to use the 3rd finger on the third frets of the 4th, 5th, and 6th strings. After this success, I now use Sor’s 4th finger approach for all my students (and myself) with the rare exception for that student with very large hands with long fingers that do not pronate with the 3rd finger on the 3rd fret.
THE SCALES HAVE BEEN LIFTED FROM MINE EYES!!! I believe these two paragraphs are the most helpful ones I've ever read on the site. Thank you, Larry!
By the way, I was a private student of Fred Noad's for almost three years, in the days before he became so well-known. I had no idea there was even such a thing as a fourth-finger approach. Or a third=finger approach, for that matter. He just taught the third finger as something natural.