quartet has 13 distinct synonyms spread across 5 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.
the cardinal number that is the sum of three and one
four performers or singers who perform together
a set of four similar things considered as a unit
four people considered as a unit
“he joined a barbershop quartet”
Synonyms, antonyms and the broader and narrower relations for quartet come from Princeton University’s WordNet. Full sources and licences.