Malachi 4 – Skeptic's Annotated Bible answered

1 For, behold, the day cometh, that shall burn as an oven; and all the proud, yea, and all that do wickedly, shall be stubble: and the day that cometh shall burn them up, saith the LORD of hosts, that it shall leave them neither root nor branch.

(3:1; 4:1, 5) "The day cometh"
The gospel of Mark claims that John the Baptist fulfilled the prophecy given in Malachi. But the Malachi prophecy says that God will send Elijah before "the great and dreadful day of the LORD" in which the world will be consumed by fire. Yet John the Baptist flatly denied that he was Elijah (Elias) in John 1:21 and the earth was not destroyed after John's appearance.

On John the Baptist, see John 1:21.
John Gill explains that this verse is not about Jesus' second coming:

Not the day of judgment, as Kimchi and other interpreters, both Jewish and Christian, think; but the day of Christ's coming in his kingdom and power, to take vengeance on the Jewish nation, which burned like an oven, both figuratively and literally; when the wrath of God, which is compared to fire, came upon that people to the uttermost; and when their city and temple were burnt about their ears, and they were surrounded with fire, as if they had been in a burning oven.

2 But unto you that fear my name shall the Sun of righteousness arise with healing in his wings; and ye shall go forth, and grow up as calves of the stall.

3 And ye shall tread down the wicked; for they shall be ashes under the soles of your feet in the day that I shall do this, saith the LORD of hosts.

(4:1-3) God will burn "the wicked" and the "righteous" will walk around on their ashes.

No! Wickedness is done away with completely, can't have that!!

4 Remember ye the law of Moses my servant, which I commanded unto him in Horeb for all Israel, with the statutes and judgments.

"Remenber ye the law of Moses."
Must we obey the Old Testament laws?
Where did Moses receive the ten commandments?

On if we must obey the Old Testament laws, see Old Testament Laws.
On where Moses received the ten commandments, see 1 Kg. 8:9.

5 Behold, I will send you Elijah the prophet before the coming of the great and dreadful day of the LORD:

(3:1; 4:1, 5) "The day cometh"
The gospel of Mark claims that John the Baptist fulfilled the prophecy given in Malachi. But the Malachi prophecy says that God will send Elijah before "the great and dreadful day of the LORD" in which the world will be consumed by fire. Yet John the Baptist flatly denied that he was Elijah (Elias) in John 1:21 and the earth was not destroyed after John's appearance.
(4:1-3) God will burn "the wicked" and the "righteous" will walk around on their ashes.

The meaning is not that Elijah himself, in the body, would return, but a person endowed with the same spirit. See John 1:21.
The dreadful day of the LORD is Jesus first coming, not his second. At that time the world shall not be consumed with fire as the author of the SAB has it, but as is written in the next verse (verse 6), the hearts of the fathers shall he turn to the children.

6 And he shall turn the heart of the fathers to the children, and the heart of the children to their fathers, lest I come and smite the earth with a curse.

The Old Testament ends fittingly with these words: "Lest I come and smite the earth with a curse." Will God curse the earth (again)?
How should parents be treated?

Looking Unto Jesus already notes that in this verse a different Hebrew word is used then in Gen. 8:21. But also the target is different. In Gen. 8:21 it is about the globe, the planet. This is about unbelievers, persons or nations.
This warning was addressed to the Jews, and they did not heed it. And it happened as forewarned. And so they have been exiled till the end of the times of the Gentiles.
On treating of parents: this verse is not about the relationship between parents and children, but the meaning is that both should be converted to the true knowledge and faith in the Messiah. And both would befall this curse if they would not heed it.