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

fourth gear.

a fourth part, esp. of one ( 1 / 4 ).

the fourth member of a series.

Music .

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 prec. by the ).

in the fourth place; fourthly.