Cambridge Bible Commentary:
“Before His 2nd Advent shall another Elijah come ‘and shall restore all things’ (Mat. 17:10-13).”

The Pulpit Commentary:
(Malachi 4) “…is not the same personage as the ‘messenger’ in Malachi 3; for the latter comes before the first advent of the Lord, and the former appears before the day of judgment.”

Barnes’ Commentary:
“He implies a coming of Elijah, other than that of John the Immerser, since he was already martyred, and all things were not yet restored.”