Malachi 4:4

This is doubtless intended for a solemn conclusion, not only of this prophecy, but of the canon of the Old Testament, and is a plain information that they were not to expect any more sayings nor writing by divine inspiration, any more of the dictates of the Spirit of prophecy, till the beginning of the gospel of the Messiah, which sets aside the Apocrypha as no part of holy writ, and which therefore the Jews never received. Now that prophecy ceases, and is about to be sealed up, there are two things required of the people of God, that lived then: - I. They must keep up an obedient veneration for the law of Moses (Mal 4:4): Remember the law of Moses my servant, and observe to do according to it, even that law which I commanded unto him in Horeb, that fiery law which was intended for all Israel, with the statutes and judgments, not only the law of the ten commandments, but all the other appointments, ceremonial and judicial, then and there given. Observe here, 1. The honourable mention that is made of Moses, the first writer of the Old Testament, in Malachi, the last writer. God by him calls him Moses my servant; for the righteous shall be had in everlasting remembrance.

— Matthew Henry's Concise Commentary (public domain)

Remember . . . law--"The law and all the prophets" were to be in force until John (Mat 11:13), no prophet intervening after Malachi; therefore they are told, "Remember the law," for in the absence of living prophets, they were likely to forget it. The office of Christ's forerunner was to bring them back to the law, which they had too much forgotten, and so "to make ready a people prepared for the Lord" at His coming (Luk 1:17). God withheld prophets for a time that men might seek after Christ with the greater desire [CALVIN]. The history of human advancement is marked by periods of rest, and again progress. So in Revelation: it is given for a time; then during its suspension men live on the memories of the past. After Malachi there was a silence of four hundred years; then a harbinger of light in the wilderness, ushering in the brightest of all the lights that had been manifested, but short-lived; then eighteen centuries during which we have been guided by the light which shone in that last manifestation. The silence has been longer than before, and will be succeeded by a more glorious and awful revelation than ever. John the Baptist was to "restore" the defaced image of "the law," so that the original might be recognized when it appeared among men [HINDS].

— Jamieson-Fausset-Brown Commentary (public domain)