And I turned, and lifted up mine eyes, and looked, and, behold, there came four chariots out from between two mountains; and the mountains were mountains of brass. There came four chariots - Four monarchies or empires. This is supposed to mean the same with the vision of the four horns, in chap. 1.
Mountains of brass - The strong barriers of God's purposes, which restrained those powers within the times and limits appointed by Jehovah.