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]