In Isaiah 14:4, Israel is told to sing a song of contempt against which king?

Answer: The king of Babylon

When the LORD gives rest, Israel will taunt the king of Babylon: "How the oppressor has ceased, and how his fury has ended!"

“On the day that the LORD gives you rest from your pain and torment, and from the hard labor into which you were forced, you will sing this song of contempt against the king of Babylon: How the oppressor has ceased, and how his fury has ended!”— Isaiah 14:3-4 (BSB)