What does Mark 9:4 mean?

In the Old Testament law, two witnesses were required to prove someone's guilt (Deuteronomy 17:6; 19:15). Now, God sends two witnesses to validate Jesus' identity. Both Moses and Elijah represent how Jesus' sacrifice and resurrection fulfill the requirements of Judaism (Matthew 5:17). Jesus satisfies the sacrifices and feasts of the Mosaic Law with His death and resurrection. He also fulfills the many Old Testament prophecies of the Messiah. Luke 9:31 says they spoke of Jesus' "departure, which he was about to accomplish at Jerusalem." "Departure" is from the Greek root word exodos and can mean leaving a situation or a place, finishing and leaving a job, or dying. Jesus eventually does all three.

Moses is one of the most important figures in Jewish history. He was born in the tribe of Levi (Exodus 2:1) but raised by the Pharaoh's daughter in Egypt (Exodus 2:10) while the other Israelites were enslaved (Exodus 1:8–14). God chose Moses to face down the Pharaoh and lead the Israelites out of Egypt and to the Promised Land (Exodus 3:7–12). He is the author of most of the first five books of the Bible and he received the Ten Commandments from God (Exodus 20:1–17). Because of his presence here, some think Moses will be one of the two witnesses in the tribulation (Revelation 11:3–13). Others think it will be Enoch, who along with Elijah didn't die but was taken to heaven by God (Genesis 5:24).

Elijah was perhaps the most prominent prophet in the Old Testament. He spent most of his ministry challenging King Ahab and his evil wife Jezebel who ruled the northern kingdom of Israel from about 874 to 853 BC. It was Elijah who challenged the pagan priests to see if Baal or God would respond to their offerings. Baal didn't show, and God consumed Elijah's offering, the wood, and the stones he'd used to make the altar (1 Kings 18). Instead of dying, Elijah was taken to heaven in a chariot of fire (2 Kings 2:11), and Malachi prophesied that he would return before the day of the Lord (Malachi 4:5), which makes him scholars' leading candidate for the role of one of the two prophets in the tribulation.