The Collaborative International Dictionary of English v.0.48:

Quadruplet \Quad"ru*plet\, n. [From Quadruple.]
   1. A collection or combination of four of a kind.
      [Webster 1913 Suppl.]

   2. pl. Four children born in the same labor.
      [Webster 1913 Suppl.]

   3. A cycle for carrying four riders, so arranged that all the
      riders can assist in the propulsion.
      [Webster 1913 Suppl.]

WordNet (r) 3.0 (2006):

quadruplet
    n 1: the cardinal number that is the sum of three and one [syn:
         four, 4, IV, tetrad, quatern, quaternion,
         quaternary, quaternity, quartet, quadruplet,
         foursome, Little Joe]
    2: one of four children born at the same time from the same
       pregnancy [syn: quadruplet, quad]
    3: a set of four similar things considered as a unit [syn:
       quartet, quartette, quadruplet, quadruple]