Fourfold.

Originally: designating a system of telegraphy enabling four messages to be sent over a wire at the same time, two in each direction (now chiefly historical). Later also: relating to or designating the transmission or reception of four signals simultaneously over one channel or with one aerial.

Of a polyploid individual: having the dominant allele of a particular gene represented four times.

A quadruplex telegraph.

A house or apartment that has four rooms.

An apartment that has four floors connected by interior staircases.

A building divided into four separate residences; = "quadplex".

To make (a circuit, conductor, etc.) quadruplex; to multiply (a circuit's capacity) fourfold; now chiefly historical.