fourth (Adjective) — Coming next after the third and just before the fifth in position, time, degree or magnitude. ex. "the fourth period of geologic time extends from the end of the tertiary period to the present"
fourth (Noun) — Following the third position; number four in a countable series.
fourth (Noun) — One of four equal parts. ex. "a fourth of a pound"
fourth (Noun) — The musical interval between one note and another four notes away from it.
fourth (Adverb) — In the fourth place.