Having mentioned the faithfulness of Chief Captain Moroni and then Alma and the sons of Mosiah, Mormon picks up on the theme of the spread of the Nephite religion. He had spoken of Helaman’s efforts in Alma 46:38. At that time, Mormon had indicated that they had arranged the affairs of the church and that their work would last for four years. At this time, it is less than a year after that statement. Thus, the four years mentioned in verse 20 are the very same four years Mormon indicated in Alma 46:38. Just as Alma 46:37 noted that they only had peace until “nearly the end of the nineteenth year of the reign of the judges,” so in verse 21 does Mormon repeat that by the end of the nineteenth year they “were compelled reluctantly to contend with their brethren, the Lamanites.”
The very specific repetition suggests that we are seeing a case of repetitive resumption (see comments accompanying 1 Nephi 6:1–6 for details of the technique). In between these two repeated phrases is mostly the story of Amalickiah among the Lamanites, which would be appropriately seen as an insertion in Mormon’s intended text. The ending of that insertion focuses on the defensive fortifications, but also includes Mormon’s insertion of his admiration for Chief Captain Moroni as well as the explanation of the justification of defensive warfare.
It would appear that Mormon has gotten back on track of his intended story, which is the war with the Lamanites that begins in the ending of the nineteenth year, and which begins officially in the beginning of our Chapter 49, although there is no intervening chapter break in the 1830 edition.