Lightner says Joseph was commanded to begin plural marriage in 1834 [?] and was threatened by an angel.
Mary Elizabeth Rollins Lightner, Letter to Emmeline B. Wells, Summer 1905, L. Tom Perry Special Collections, Harold B. Lee Library, Brigham Young University
he said I was the first woman God commanded him to take as a plural wife, \in 1831/ [?] he was very much frightened about it until the Angel appeared to him three times. it was in the early part of Feb, 1842 before that he was compelled to reveal it to me personally, by the Angel threatening him
We have chosen to render "1831" as such because it is difficult to determine the final number. It is possible that she originally wrote "1" and then attempted to correct it to "4." (The date 1834 would match other sources from Lightner.) Then again, it may not be a number at all. The date is not definitive in this document.