next after the third; being the ordinal number for four.
being one of four equal parts.
Automotive. of, pertaining to, or operating at the gear transmission ratio at which the drive shaft speed is greater than that of third gear for a given engine crankshaft speed, but not so great as that of fifth gear, if such exists:
a fourth part, especially of one (¼).
the fourth member of a series.
a tone on the fourth degree from a given tone (counted as the first).
the interval between such tones.
the harmonic combination of such tones.
Automotive. fourth gear:
She downshifted from fifth to fourth as we started up the hill.
(initial capital letter) Independence Day; the Fourth of July (usually preceded by the).