The four horns which have scattered Judah and Jerusalem (Israel should probably be omitted) represent the whole world arrayed against Judah, and are perhaps iron horns like those made by Zedekiah (1 Kings 22:11); hence smiths are introduced to shatter them.
Related Commentaries of Zechariah 1:18
Zechariah 1:18-21
18 Then lifted I up mine eyes, and saw, and behold four horns.
19 And I said unto the angel that talked with me, What be these? And he answered me, These are the horns which have scattered Judah, Israel, and Jerusalem.
20 And the LORD shewed me four carpenters.
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.