Hi,
Simply stated 4wire connection means send and receive use separate paths and nothing more. How many wires are physically used, depends on the application.
As I have given above a 4-wire RS485 may reqire a cable having 4pairs of twisted wires, one pair for the differential TX signal from master to all receivers of slaves , second pair connecting differental TX from all slaves to RX input of Master, third pair (both wires of the pair) used to connect ground signal of the master to ground signals of all slaves. The fourth pair is simply kept as a spare. In addition, cables used for industrial applications may have an overall shield connected to ground at one point and an armouring ( braided metal conduit) used to provide mechanical strength to the cable as well as to serve as a second grounded shield to protect against radiated noise pickups.
Regards,
Laktronics