Finally, there will be a fourth kingdom, strong as iron – for iron breaks and smashes everything – and as iron breaks things to pieces, so it will crush and break all the others. (NIV)
And the fourth kingdom [Rome] shall be strong as iron, since iron breaks to pieces and subdues all things; and like iron which crushes, it shall break and crush all these. [Dan. 7:7, 23.] (AMP)
And the fourth kingdom shall be strong as iron: forasmuch as iron breaketh in pieces and subdueth all things: and as iron that breaketh all these, shall it break in pieces and bruise. (KJV)