Here we see two instances of the tendency to shorten the long expression “in the commencement of the Xth year of the reign of the judges”. The first possibility involves omitting “the commencement of”. Here in Alma 4:20, scribe 2 of 𝓟 started to write “in the ninth year” rather than “in the commencement of the ninth year”. However, he caught his error after writing “in the ninth”, crossing out the ninth and then continuing inline with “commencement of the ninth year”. For a fuller discussion of seven other instances of the tendency to omit “the commencement of” (two of which entered the printed editions), see under Alma 30:5.
The second possibility for change in this expression involves omitting the phrase “of the reign” from “the Xth year of the reign of the judges”. Here in Alma 4:20, scribe 2 of 𝓟 omitted the phrase, but Oliver Cowdery supplied it in his proofing of 𝓟 against 𝓞. There is one more example of this kind of omission in the textual history:
In this case, the typesetter for the 1849 LDS edition introduced the error, which has been retained in all subsequent LDS editions. For further discussion regarding the possible loss of the phrase “of the reign”, see under Alma 54:1.
A third possibility, not exemplified here in Alma 4:20, is to omit the phrase “of the judges”. This occurred eight times in 𝓞 as the scribe took down Joseph Smith’s dictation. As might be expected for this phrase, the scribe caught the error in each case. For a list of those examples, see the discussion under Alma 28:7.
There is also one case where the text momentarily lost even a larger portion of the expression— namely, “of the reign of the judges” in Helaman 16:12 (see the discussion there).
The critical text will always maintain the reading of the earliest textual sources with respect to the phrase “in (the commencement of ) the Xth year of the reign of the judges”. Here in Alma 4:20, the corrected reading in 𝓟 is undoubtedly the original reading.
Summary: Maintain in Alma 4:20 the corrected reading in 𝓟: “in the commencement of the ninth year of the reign of the judges over the people of Nephi”.