Synonyms for Quadruplet (same or very similar meaning)
WordNet sense 1 (the cardinal number that is the sum of three and one):
four, foursome, quartet, IV, quatern, quaternary, quaternion, quaternity, tetrad, Little JoeWordNet sense 2 (one of four children born at the same time from the same pregnancy):
quadWordNet sense 3 (a set of four similar things considered as a unit):
quadruple, quartetteFrom the ODE community, based on WordNetadd/edit