fourth has 9 distinct synonyms spread across 3 of its 5 senses. Each sense below keeps its own list, because a synonym belongs to a meaning and not to a word: swapping one sense's substitute into another sense's sentence is how a thesaurus makes writing worse. WordNet records no antonym for it — antonymy is a relation between word forms rather than concepts, so it is deliberately sparse.
Counts are distinct word forms across every sense. A synonym that appears under three senses is counted once here and shown three times below, under each sense it belongs to.
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”
Synonyms, antonyms and the broader and narrower relations for fourth come from Princeton University’s WordNet. Full sources and licences.