What does Matthew 4:4 mean?

After Jesus fasted for 40 days in the wilderness, He was hungry. "Fasting" does not require that a person have no food, at all, for the entire period. Given that Jesus was in the wilderness, however, scholars speculate He might have gone entirely without food and only taken in water. His body would have been aching to eat something. Satan's first temptation to Jesus is designed to exploit His human, physical appetite. He challenges Jesus to turn rocks into loafs of bread if He is the Son of God.

Satan knew Jesus was God's Son, and so did Jesus. The point of the temptation—and that remark—is to goad Jesus into acting independently of God the Father. Satan's goal is to cause Jesus to sin through exercising His own will instead of the Father's. Jesus understood clearly that the Father meant for Him to endure temptations in His physically weakened condition.

Jesus resists each temptation while quoting from Deuteronomy, tying His own experience of 40 days in the wilderness to Israel's experience of the 40 years after their escape from Egypt. Here, Jesus quotes from Deuteronomy 8. In that passage, Moses is addressing the Israelites just as they are preparing to go finally enter the Promised Land of Israel:

"You shall remember the whole way that the LORD your God has led you these forty years in the wilderness, that he might humble you, testing you to know what was in your heart, whether you would keep his commandments or not. And he humbled you and let you hunger and fed you with manna, which you did not know, nor did your fathers know, that he might make you know that man does not live by bread alone, but man lives by every word that comes from the mouth of the LORD" (Deuteronomy 8:2–3).

Jesus tells Satan in no uncertain terms that He will live on obedience to the Father's word before giving into His appetite for food.