God is a master of timing. Like every other good leader, he knows when he can take the next step and how to prepare people for that next step.
Malachi foretells a day on which God will bring justice for every person and defeat evil. God promises to send Elijah before that day comes. Like so many prophecies, this one also has two meanings. The first was fulfilled when Jesus was born and John the Baptist, as an Elijah figure, prepared the way. The ultimate fulfillment of this prophecy will come in the last days, when God sends another Elijah who prepares the way for the return of Jesus.
God practices the Law of Timing like no other. He makes sure that everything is prepared before he takes his next step. These words from God are the last words he gave before he himself came into this world in the person of Jesus.
Malachi 4:5 Behold, I will send you Elijah the prophet before the coming of the great and dreadful day of the LORD.