What does "fourth" mean?

Additional senses

1. 2.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. 3.(Mus.) The interval of two tones and a semitone, embracing four diatonic degrees of the scale; the subdominant of any key. The Fourth, specifically, un the United States, the fourth day of July, the anniversary of the declaration of American independence; as, to celebrate the Fourth.