noun (n)

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

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

  source: wordnet30
• one of four children born at the same time from the same pregnancy(noun.person)

  Synonym:
  quad
  

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

  Synonym:
  quadruple, quartet, quartette
  

  source: wordnet30
• A collection or combination of four of a kind.(noun)

  source: webster1913