Definitions of fourth:

• noun:   the musical interval between one note and another four notes away from it
• noun:   following the third position; number four in a countable series
• noun:   one of four equal parts
• adjective:   coming next after the third and just before the fifth in position or time or degree or magnitude
• adverb:   in the fourth place

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