4 conductor wire is good for bi-wiring and bi-amping speakers. It is also good for the prewire phase when running ceiling speakers to volume controls and to another speaker. From the VC a 16/4 (16AWG,4-conductor)cable can go to one speaker, then split off to a 16/2 to the other speaker.
Bi-wiring would be when you would use 4-conductor wire to wire a single speaker from an amp. If the speaker has two sets of red and black terminals, one to the highs and one to the mids, or lows. Bi-amping would use these same two speaker terminals, but use two amps. Many audiophiles agree that bi-wiring, or bi-amping makes the sound cleaner.
This is often used for in-wall installations too, especially for surround speakers. You run a 4 conductor cable from the front of the room to the back of the room and hook it up to banana plug wall plates on either side.