Source : Webster's Revised Unabridged Dictionary (1913)

Fourth \Fourth\, a. [OE. fourthe, ferthe, feorthe, AS.
   fe['o]r[eth]a, fr. fe['o]wer four.]
   1. Next in order after the third; the ordinal of four.

   2. Forming one of four equal parts into which anything may be
      divided.

Fourth \Fourth\, n.
   1. 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.

   2. (Mus.) The interval of two tones and a semitone, embracing
      four diatonic degrees of the scale; the subdominant of any
      key.

   {The Fourth}, specifically, in the United States, the fourth
      day of July, the anniversary of the declaration of
      American independence; as, to celebrate the Fourth.

Source : WordNet®

fourth
     n 1: following the third position; number four in a countable
          series
     2: one of four equal parts; "a quarter of a pound" [syn: {one-fourth},
         {quarter}, {fourth part}, {twenty-five percent}, {quartern}]
     3: the musical interval between one note and another four notes
        away from it

fourth
     adj : coming next after the third and just before the fifth in
           position or time or degree or magnitude; "the
           quaternary period of geologic time extends from the end
           of the tertiary period to the present" [syn: {4th}, {quaternary}]

fourth
     adv : in the fourth place; "fourthly, you must pay the rent on the
           first of the month" [syn: {fourthly}]