quadruplex system
Webster's Dictionary (GCIDE)
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]