The Collaborative International Dictionary of English v.0.48:

Quadruplex \Quad"ru*plex\, a. [L., from quattuor four + plicare
   to fold.]
   Fourfold; folded or doubled twice.
   [1913 Webster]

   Quadruplex system (Electric Telegraph), a system by which
      four messages, two in each direction, may be sent
      simultaneously over the wire.
      [1913 Webster]