The Collaborative International Dictionary of English v.0.48:

Quadruple \Quad"ru*ple\, v. t. [imp. & p. p. Quadrupled; p.
   pr. & vb. n. Quadrupling.] [L. quadruplare: cf. F.
   quadrupler.]
   To multiply by four; to increase fourfold; to double; to
   double twice. --A. Smith.
   [1913 Webster]

WordNet (r) 3.0 (2006):

quadrupling
    n 1: increase by a factor of four