Daniel 7:19
"Then I desired to know the truth about the fourth beast, which was different from all the rest, exceedingly terrifying, with its teeth of iron and claws of bronze, and which devoured and broke in pieces and stamped what was left with its feet,
Daniel 7:7
After this I saw in the night visions, and behold, a fourth beast, terrifying and dreadful and exceedingly strong. It had great iron teeth; it devoured and broke in pieces and stamped what was left with its feet. It was different from all the beasts that were before it, and it had ten horns.
Daniel 2:40-43
And there shall be a fourth kingdom, strong as iron, because iron breaks to pieces and shatters all things. And like iron that crushes, it shall break and crush all these.