Ammaron had kept a record (see ending of v. 49), but Mormon does not abridge anything from it except his being moved by the Holy Ghost to hide up the record (v. 48). Perhaps there was nothing in those years prior to his hiding up the plates that had not already been described (such as: the wickedness and the Gadianton band). All of the records kept up to that time would have been very numerous. Thus the report of Joseph Smith, Oliver Cowdery, and others seeing wagon loads of plates in the Hill Cumorah would be reasonable.
The promises of the Lord (v. 49) would include the promise to Enos, son of Jacob, and his fathers, who would be Jacob and Lehi (see Enos 1:11–18). There were undoubtedly others (see D&C 10:45–52 also referenced above). The prophecies of the coming forth of the records were given to Mormon (see Words of Mormon 1:9–11; Mormon 5:9); and Moroni (see Mormon 8:26–32; Ether 5). Although not stated specifically, Nephi and Jacob also knew of these prophecies and promises (see Jacob 1:1–5), as well as all the other record keepers (see Words of Mormon 1:10–11).
The record of 4 Nephi covers 286 years of the Nephites occupying the land of the Americas. The first 166+ years gives us a brief overview of their building a Zion society. The next 120 years gives an account of falling from Zion to wickedness. The decline will continue and will be described in the next two chapters of this work.