Malachi 4:5

I send you Elijah--as a means towards your "remembering the law" (Mal 4:4). the prophet--emphatical; not "the Tishbite"; for it is in his official, not his personal capacity, that his coming is here predicted. In this sense, John the Baptist was an Elijah in spirit (Luk 1:16-17), but not the literal Elijah; whence when asked, "Art thou Elias?" (Joh 1:21), He answered, "I am not." "Art thou that prophet?" "No." This implies that John, though knowing from the angel's announcement to his father that he was referred to by Mal 4:5 (Luk 1:17), whence he wore the costume of Elijah, yet knew by inspiration that he did not exhaustively fulfil all that is included in this prophecy: that there is a further fulfilment (compare Note, see on Mal 3:1). As Moses in Mal 4:4 represents the law, so Elijah represents the prophets. The Jews always understood it of the literal Elijah. Their saying is, "Messiah must be anointed by Elijah." As there is another consummating advent of Messiah Himself, so also of His forerunner Elijah; perhaps in person, as at the transfiguration (Mat 17:3; compare Mat 17:11). He in his appearance at the transfiguration in that body on which death had never passed is the forerunner of the saints who shall be found alive at the Lord's second coming. Rev 11:3 may refer to the same witnesses as at the transfiguration, Moses and Elijah; Rev 11:6 identifies the latter (compare Kg1 17:1; Jam 5:17).

— Jamieson-Fausset-Brown Commentary (public domain)