What is Fourth?

Fourth definition and meaning on Dictionary terms:
next after the third; being the ordinal number for four.
being one of four equal parts.

Automotive. of, relating 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, especially of one (