Princeton's WordNet

1. fourth(noun)

   following the third position; number four in a countable series

   Synonyms:
   quartern, quarter, one-fourth, twenty-five percent, fourth part, one-quarter

   Antonyms:
   cardinal

2. one-fourth, fourth, one-quarter, quarter, fourth part, twenty-five percent, quartern(noun)

   one of four equal parts

   Synonyms:
   fourth, after part, quartern, one-quarter, fourth part, twenty-five percent, one-fourth, stern, quarter, tail, poop

   Antonyms:
   cardinal

3. fourth(adj)

   the musical interval between one note and another four notes away from it

   Synonyms:
   quartern, quarter, one-fourth, twenty-five percent, fourth part, one-quarter

   Antonyms:
   cardinal

4. fourth, 4th, quaternary(adverb)

   coming next after the third and just before the fifth in position or time or degree or magnitude

   Synonyms:
   quaternary, fourth, quaternate

   Antonyms:
   cardinal

5. fourthly, fourth(adverb)

   in the fourth place

   Synonyms:
   fourthly, fourth

   Antonyms:
   cardinal