quartet vs piece of music

quartet and piece of music both are nouns.

Word	Noun	Adjective	Verb	Adverb
quartet	Yes	No	No	No
piece of music	Yes	No	No	No

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

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

quartet (noun)	piece of music (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