Zechariah 1:21
Then said I, What come these to do? And he spake, saying, These are the horns which have scattered Judah, so that no man did lift up his head: but these are come to fray them, to cast out the horns of the Gentiles, which lifted up their horn over the land of Judah to scatter it.
The four horns are the powers that scattered Judah, Israel, and Jerusalem. That scattering kept the people bowed down and powerless, so that no man could lift up his head.
The four carpenters are the agencies God uses to terrify them, to drive out those hostile powers and to overthrow them. By them He restores His people and the house and place of worship. That restoration comes both through God's own action and through the decrees of Cyrus and his successors.