The context of the final verse
Malachi closes by calling Israel to remember the law of Moses and by promising the coming of Elijah before the great and dreadful day of the LORD. The final verse describes a work of turning hearts so that judgment does not fall upon the land.
The New Testament opens with this expectation still active. Luke 1 connects the language of Malachi with the ministry of John the Baptist, who prepares a people for the Lord.
Why Bible order matters
Malachi 4:6 is the final Old Testament verse in the ordering used by the KJV and most English Christian Bibles. The Hebrew Bible arranges its books differently and ends with 2 Chronicles, even though it contains the same Old Testament books in a different order.
For that reason, “the last verse of the Old Testament” can refer either to canonical order in an English Bible or to the ordering of the Hebrew Scriptures. BibleAV follows the KJV order.