Co je fourth? Co je fourth? 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.