quartet vs quaternity

quartet and quaternity both are nouns.

Word	Noun	Adjective	Verb	Adverb
quartet	Yes	No	No	No
quaternity	Yes	No	No	No

As nouns, quartet and quaternity are synonyms defined as:

quartet and quaternity: the cardinal number that is the sum of three and one

Other synonyms of quartet include 4, four, foursome, IV, Little Joe, quadruplet, quatern, quaternary, quaternion, tetrad.

quartet (noun)	quaternity (noun)
a musical composition for four performers	the cardinal number that is the sum of three and one
four people considered as a unit
a set of four similar things considered as a unit
four performers or singers who perform together
the cardinal number that is the sum of three and one