So, Two questions.
1. Can the 4th speaker volume be controlled individually?
2. I guess if it can then I can assume it can also play something different than the downstairs speakers. e.g. TV Downstairs and music upstairs, or off upstairs if required.
Thanks in advance for any advice.
Best answer by pwt
It's possible you might be able to insert some form of volume control and on/off switching in the wiring between the Amp and your speakers, but this would be outside Sonos control. To control the speakers as different Sonos rooms, you'll need additional Amps, one per room that you want to control independently.
There are alternative solutions for multi-room amplification, including multi-channel amps with multiple Sonos Connects.