The record on the brass plates of Laban was larger than our Old Testament (1 Nephi 13:23). It included prophecies from Zenos, Zenock, Neum and probably many others. Robert Millet and Robert Nyman make the argument that the promise given in verse 4 implies that the lost portions and mysteries of the Old Testament will someday be made available to us. With our current version of the Old Testament, we have many of these mysteries but not all of them
"Does anyone think we have all of the words of Isaiah or Jeremiah or Malachi? And are there not prophets and apostles without number, whose names we do not even know, who have recorded their teachings and testimonies?
"The perfected Bible of the future will surely include all that was on the brass plates of Laban. Indeed, Lehi prophesied ’that these plates of brass should go forth unto all nations, kindreds, tongues, and people who were of his seed. Wherefore, he said that these plates of brass should never perish; neither should they be dimmed any more by time.’ (1 Nephi 5:18–19.)
"More than five hundred years later Alma testified that they should ’be kept and preserved by the hand of the Lord until they should go forth unto every nation, kindred, tongue, and people, that they shall know of the mysteries contained thereon,‘ and that they would ’retain their brightness.’ (Alma 37:4–5.)
"Someday the Lord will raise up a prophet, who will also be a seer and a translator, to whom he will give the brass plates that they may be translated for the benefit and blessing of those in all nations.
“Would God that the work might commence at least in our day, though in fact we have no such hope. Why should the Lord give us what is on the brass plates or in the sealed portion of the Book of Mormon when we do not even treasure up and live by what he has already given us?” (Monte Nyman & Robert Millet, The Joseph Smith Translation: The Restoration of Plain and Precious Things, p. 15-6)