Daniel 7:19
"Then I wanted to know the meaning of the fourth beast, which was different from all the others. It was very dreadful, with two rows of iron teeth and bronze claws, and it devoured, crushed, and trampled anything that was left with its feet.
Daniel 7:7
"After these things, as I was watching in the night visions a fourth beast appeared -- one dreadful, terrible, and very strong. It had two large rows of iron teeth. It devoured and crushed, and anything that was left it trampled with its feet. It was different from all the beasts that came before it, and it had ten horns.
Daniel 2:40-43
Then there will be a fourth kingdom, one strong like iron. Just like iron breaks in pieces and shatters everything, and as iron breaks in pieces all of these metals, so it will break in pieces and crush the others.