noun

• (in the singular) The person or thing in the fourth position.
  "We are one F and two M's seeking the company of a fourth (M or F) for our spacious and friendly Somerville apt."
• (chiefly US) A quarter, one of four equal parts of a whole.
• (in the singular) The fourth gear of an engine.
• (music) A musical interval which spans four degrees of the diatonic scale, for example C to F (C D E F).
  "Now I've heard there was a secret chord / That David played, and it pleased the Lord / But you don't really care for music, do ya? / It goes like this, the fourth, the fifth"

verb

• (informal) To agree with a proposition or statement after it has already been thirded.
  "If he remembered rightly what took place in the House on the first night of the session, the Attorney General would have to file an information, not only against the noble Lord who proposed the address, and the hon. Member for London who seconded it, but also against the hon. Member who thirded, and the noble Lord who fourthed it, and indeed against every Member in the House."

name

• (US) The Fourth of July.