So, as I’ve started reading the Book of Mormon again, I wondered why, in the heading Nephi wrote to the book of 1st Nephi, which obviously is written after they have taken the ship and landed in the promised land, he started the summary only listing 4 sons of Lehi, when in fact at the time of writing his younger siblings had been born.

The only explanation I can give is that he started his record abridging content from other records, rather than starting from a completely blank sheet of paper to introduce his family, and his other records begin with just four sons. An interesting question to me, with perhaps broader implications than what I’ve discovered.