BC 4 specifies that Joseph is to “translate a few more pages” and then stop for a season. By the end of March, 1829 – Smith had likely completed translation of Mosiah, with Emma and Reuben as scribes. Note the absence of phrases from Mosiah. Note also the absence of phrases the Book of Moroni. The Book of Moroni was likely dictated in June 1830, at the completion of dictation.
The phrases used in BC4 come from Book of Mormon chapters attributed to many authors, including Spalding. This suggests that Smith composed BC 4 using phrases from text then in his possession. BoM chapter attributions for unique phrases in BC 4:
Rigdon – 45%.
Spalding – 22%
Cowdery –9%
Smith – 9%
Pratt – 13%
Rigdon’s late-period revelations in the Post Collection (University of Utah) contain BC4 phrases, including “mighty prayer” 2 times,” gotten” 5 times, “wo” 11 times, “establish my” 1 time, “all manner of” 2 times, “manifestations” 1 time.