Here the 1874 RLDS edition accidentally omitted the word again. Note that the following word is against, which means that the typesetter’s eye must have skipped across again because of its near identity to against. For other examples of this error in the history of the text, see under Alma 44:19–20 and 3 Nephi 2:18.
In the 1906 LDS edition, the word order for come again was switched to again come. This change was not transferred to any subsequent LDS edition since the 1906 edition never served as a copytext. Elsewhere, the text definitely prefers the word order “come again” over “again come” (34 to 2). But either order is possible, so we follow in each case the earliest reading, thus “the Lamanites did not come again against the Nephites” here in Mormon 4:16.
Summary: Maintain in Mormon 4:16 the original again as well as its placement after the verb come.