Fourth definition

(1) Coming next after the third and just before the fifth in position or time or degree or magnitude

"The quaternary period of geologic time extends from the end of the tertiary period to the present"

(2) In the fourth place

"Fourthly, you must pay the rent on the first of the month"

(3) The musical interval between one note and another four notes away from it

(4) One of four equal parts

"A quarter of a pound"

(5) Following the third position; number four in a countable series