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: Webster's Revised Unabridged Dictionary (1913)