The ordinal number matching the number four in a series. One of four equal parts. Music A tone four degrees above or below a given tone in a diatonic scale. Music The interval between two such tones. Music The harmonic combination of these tones. Music The subdominant of a scale or key. The transmission gear or gear ratio used to produce forward speeds next higher to those of third in a motor vehicle. The Fourth of July; Independence Day. Used with the: went to a parade on the Fourth.