The Collaborative International Dictionary of English v.0.48:

Quadrennial \Quad*ren"ni*al\, a. [L. quadriennium a space of
   four years; quattuor four + annus year; cf. L. quadriennis.
   See Quadrate, and Annual.]
   1. Comprising four years; as, a quadrennial period.
      [1913 Webster]

   2. Occurring once in four years, or at the end of every four
      years; as, quadrennial games.
      [1913 Webster]