quartet vs opus

quartet and opus both are nouns.

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

As nouns, opus is a hypernym of quartet; that is, opus is a word with a broader meaning than quartet:

Other hypernyms of quartet include composition, musical composition, piece, piece of music.

quartet (noun)	opus (noun)
a musical composition for four performers	a musical work that has been created
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