Each node can communicate independently. In the two-wire network the transmitter and receiver of each device are connected to one single twisted pair. All devices are connected to a single twisted pair.
Communication goes over the single line in both directions.
Data transmission from more than one device must be prevented by software. Four-wire networks have one master port, there the transmitter is connected to each of the slave receivers on one twisted pair. The slave transmitters are all connected to the master receiver on the second twisted pair. The master does not need to have a tri-state output, since the slave devices are all transmitting over the second twisted pair. These second pair is intended only to send data from slaves to the master.