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Definition of fourth:
 Forming one of four equal parts into which anything may be divided.
 Next in order after the third; the ordinal of four.
 One of four equal parts into which one whole may be divided; the quotient of a unit divided by four; one coming next in order after the third.
 The interval of two tones and a semitone, embracing four diatonic degrees of the scale; the subdominant of any key.
Synonyms:
quaternate, onequarter, fourth part, tail, onefourth, fourthly, quaternary, ordinal, poop, stern, quarter, twentyfive percent, after part, 4th, quartern.
Usage examples:

Then these two would invite a third man, possibly even a fourth, to have lunch with them and talk in a general way, even talk of other things part of the time.
 "Sunshine Sketches of a Little Town", Stephen Leacock. 
On the twenty fourth all was ready to bring home the boat.
 "A Singer from the Sea", Amelia Edith Huddleston Barr. 
Give me a chance, 'n' if I'm no good as third mate, I'll probably do as fourth.
 "Mr. Trunnell", T. Jenkins Hains.