Special circuits, such at T1s used to be two separate pairs,one for transmit and one for receive. This is not usually the case anymore as newer HDSL technology allows them to be combined on one pair. At the customer's premises, they are still broken out into separate transmit and receive circuits.

Four wire circuits were also used for voice in foreign exchange environments where a telephone number may be coming from a distant CO in another city or even another state. Again, with today's digital technology, this isn't a requirement anymore. Back in the day, transmit and receive were kept separate in order to overcome longer distances than possible with a traditional two wire loop.

Ed Vaughn, MBSWWYPBX