As explained under Helaman 16:10, in the latter part of the text the 1830 typesetter usually removed the Hebraistic and, as here in Mormon 3:4 where the and preceded the main clause “the king of the Lamanites sent an epistle unto me” and separated it from the preceding subordinate after-clause with its associated present participial clause. The critical text will restore this original instance of and.
Summary: Restore in Mormon 3:4 the Hebraistic and that originally occurred just before the main clause “the king of the Lamanites sent an epistle unto me”.