Here the typesetter for the 1841 British edition omitted the preposition unto. The 1849 LDS edition restored the original unto. For another example where the 1841 edition omitted unto, see under 3 Nephi 24:1. As explained under Alma 7:4, the Book of Mormon text prefers the unto in the verb phrase “to give (unto) someone to know”.
Summary: Maintain in Mormon 3:4 the preposition unto in the relative clause “which gave unto me to know that …” (the reading of the earliest text).