The 1841 British edition accidentally replaced the plural depths with the singular depth. The correct depths was restored in the subsequent LDS edition (1849). Elsewhere the original text has only the plural usage, “the depths of humility” (five more times). For discussion, see under 1 Nephi 8:32 and Alma 62:41.
Summary: Maintain the plural “the depths of humility”, here in Mosiah 4:11 and elsewhere in the text.