Source : Webster's Revised Unabridged Dictionary (1913)

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

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

Source : WordNet®

quadruplex
     adj : having four units or components; "quadruple rhythm has four
           beats per measure"; "quadruplex wire" [syn: {quadruple},
            {quadruplicate}, {fourfold}]

Source : Free On-Line Dictionary of Computing

quadruplex
     
         A kind of telegraphy system developed by
        Thomas A. Edison in the 1870s combining {diplex} and {duplex}
        communications to support simultaneous transmission of two
        messages in each direction.
     
        (2000-04-02)