John Wesley on Zechariah 6:1

Four chariots - Angels who are sometimes styled chariots of God. These as employed in the affairs of church and empire, act their part in the revolution and changes of things, 'till the gospel be preached by the Messiah, and the apostles. Of brass - These denote the immoveable decrees of God, his steady execution of his counsels and the insuperable restraints upon all empires and countries, which God keeps within the barriers of such impregnable mountains.