Synonyms for Quartet (same or very similar meaning)
WordNet sense 1 (the cardinal number that is the sum of three and one):
four, foursome, quadruplet, IV, quatern, quaternary, quaternion, quaternity, tetrad, Little JoeWordNet sense 2 (four performers or singers who perform together):
quartetteWordNet sense 3 (a set of four similar things considered as a unit):
quadrupleFrom the ODE community, based on WordNetadd/edit