Isaiah 3:10 (quoted in 2 Nephi 13:10) has Jehovah command Isaiah: “Say unto the righteous that it is well with them; for they shall eat the fruit of their doings.” Nephi has the same message for those who live at the time of the great destructions preceding the Messiah’s appearance. Those who will be righteous and look forward to the Messiah will not perish in that terrible day. It is intended to be a parallel promise to the one Jehovah commanded that Isaiah declare.
The Book of Mormon uses the title Son of Righteousness as a name for the Messiah. Although it is similar to the phrase Sun of righteousness in Malachi 4:2, the Malachi version is sun, spelled s-u-n, and Nephi’s version indicates the s-o-n. The idea that the Messiah would heal them is both accurate prophecy of what will happen when the Messiah arrives, but also a possible allusion to Malachi 4:2, which says that “the Sun of righteousness [will] arise with healing in his wings.”
The final phrase is an important timed prophecy. After the Messiah appears, there will be four hundred years pass away in righteousness. That four hundred years accurately predicts the timing of the destruction of the Nephites, but does not accurately predict that the whole time will be righteous. That is because the timing of the righteousness is not the point. The point is the four hundred years.
Assuming that the Book of Mormon is taking place in Mesoamerica as Nephi wrote, a four-hundred-year prophecy would have been particularly significant. Mesoamericans used a base 20 method of counting, and their four-hundred-year period was called, in Maya, a baktun. It had the kind of psychic force that a thousand years does for us, who are used to a base 10 system.