adjective
The ordinal form of the number four.
noun
(in the singular) The person or thing in the fourth position.
(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).
(astronomy, rare) One sixtieth of a third or tierce.
verb
(informal) To agree with a proposition or statement after it has already been thirded.