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]

WordNet (r) 3.0 (2006):

quadruplex
    adj 1: having four units or components; "quadruple rhythm has
           four beats per measure"; "quadruplex wire" [syn:
           quadruple, quadruplicate, quadruplex, fourfold,
           four-fold]

The Free On-line Dictionary of Computing (30 December 2018):

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)