The final two verses promise that the great prophet Elijah (who was carried into heaven by a chariot of fire, and thus never died) will return to make a final attempt to help the people become truly faithful -- so that God will reward them instead of destroy them on the Day of the Lord. The promise of Elijah's return has been a key part of Jewish faith for almost 2500 years -- and is still a prominent theme in the Jewish Passover seder.
For Christians, this promise is important too because it shapes how people responded to Jesus during his life. When people wonder if Jesus is Elijah, it means that they are asking if Jesus is God's chosen messenger, sent to teach and redeem the people from their sinful and unfaithful ways. (It turns out that Jesus is more than the prophet Elijah, which is what the 27 books of the New Testament explore in detail, staring with the next book of Godsway 66, Matthew.)
[links to Bible Gateway open in a new window]Reading the Bible in Its Entirety
If you are planning to read the Bible in its entirety, you should read Malachi 4 today.