The Collaborative International Dictionary of English v.0.48:

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.
      [1913 Webster]

   2. Forming one of four equal parts into which anything may be
      divided.
      [1913 Webster]

The Collaborative International Dictionary of English v.0.48:

Fourth \Fourth\, n.
   1. One of four equal parts into which one whole may be
      divided; the quotient of a unit divided by four.
      [1913 Webster]

   2. (Mus.) The interval of two tones and a semitone, embracing
      four diatonic degrees of the scale; the subdominant of any
      key.
      [1913 Webster]

   3. One coming next in order after the third.
      [1913 Webster]

   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.
      [1913 Webster]

WordNet (r) 3.0 (2006):

fourth
    adv 1: in the fourth place; "fourthly, you must pay the rent on
           the first of the month" [syn: fourthly, fourth]
    adj 1: 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: fourth, 4th,
           quaternary]
    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, fourth, one-quarter, quarter, fourth part,
       twenty-five percent, quartern]
    3: the musical interval between one note and another four notes
       away from it

Moby Thesaurus II by Grady Ward, 1.0:

36 Moby Thesaurus words for "fourth":
   consecutive intervals, degree, diapason, diatessaron,
   diatonic interval, diatonic semitone, enharmonic diesis,
   enharmonic interval, farthing, fifth, fourth part, half step,
   halftone, interval, less semitone, melodic interval, note, octave,
   one-fourth, parallel octaves, quart, quarter, quartern, quarto,
   second, semitone, seventh, sixth, step, third, tone,
   twenty-five cents, twenty-five percent, two bits, unison interval,
   whole step