noun (n)

• the cardinal number that is the sum of three and one(noun.quantity)

  Synonym:
  4, four, foursome, iv, little joe, quadruplet, quatern, quaternary, quaternion, quaternity, tetrad
  

  source: wordnet30
• four performers or singers who perform together(noun.group)

  Synonym:
  quartette
  

  source: wordnet30
• a set of four similar things considered as a unit(noun.group)

  Synonym:
  quadruple, quadruplet, quartette
  

  source: wordnet30
• four people considered as a unit(noun.group)

  Example:
  He joined a barbershop quartet.
  

  Synonym:
  foursome, quartette
  

  source: wordnet30
• a musical composition for four performers(noun.communication)

  source: wordnet30
• A composition in four parts, each performed by a single voice or instrument.(noun)

  source: webster1913