The 1905 LDS edition accidentally added the adverb up here in Mormon 4:15. The 1911 LDS edition followed this reading, but the 1920 LDS edition restored the earlier reading to the LDS text. This use of up is consistent with the descriptions of battle movement in Mormon. Usually, the text in Mormon uses neither up nor down in describing the various attacks (18 times); but when it does, the Lamanites always “come down” against the Nephites while the Nephites always “go up” against the Lamanites. There are nine examples:
(For six other examples in the text where “come down” is used to refer to Lamanite attacks against the Nephites, see under Alma 46:30.) Still, in the majority of cases here in Mormon no vertical direction is mentioned, as in the example under discussion and the example in the subsequent verse:
Although the addition of up in the 1905 edition for Mormon 4:15 turns out to be appropriate for the verb go (the Nephites are attacking the Lamanites), the critical text will maintain the reading of the earliest text.
Summary: Maintain the earliest text in Mormon 4:15 without the adverb up; the insertion of up in the 1905 LDS edition appears to be accidental, even if consistent with usage elsewhere in Mormon.