The time of preparation is four years. The example reported by Ixtlilxochitl allowed ten years, so four years does not seem foreign to Mesoamerican practice.
Chronology: Mormon 5:6 gave the last year-date as the 380th year. At this point, “three hundred and eighty four years had passed away”; and we are in the 385th, which would be A.D. 376. Mormon was seventy-five.