four vs quartet

four and quartet both are nouns.

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

As nouns, four and quartet are synonyms defined as:

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

four (noun)	quartet (noun)
a playing card or domino or die whose upward face shows four pips	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

four (adjective)	quartet (adjective)
being one more than three