Daniel 2:40-43
Finally, there will be a fourth kingdom as strong as iron; for iron shatters and crushes all things, and like iron that crushes all things, it will shatter and crush all the others. . . .

Daniel 7:7, 8, 19-26
After this, as I watched in my vision in the night, suddenly a fourth beast appeared, and it was terrifying--dreadful and extremely strong--with large iron teeth. It devoured and crushed; then it trampled underfoot whatever was left. It was different from all the beasts before it, and it had ten horns. . . .