quartet in WordNet English dictionary

1. the cardinal number that is the sum of three and one
   four  4  IV  tetrad  quatern  quaternion  quaternary  quaternity  quartet  quadruplet  foursome  Little Joe 
   
2. four people considered as a unit
   "he joined a barbershop quartet"; "the foursome teed off before 9 a.m."
   quartet  quartette  foursome 
   
3. a set of four similar things considered as a unit
   quartet  quartette  quadruplet  quadruple 
   
4. a musical composition for four performers
   quartet  quartette 
   
5. four performers or singers who perform together
   quartet  quartette