Upgrade the main room to a Linn Majik DSM system with better speakers and keep the extra room options of either Linn or Sonos from the same music library
The Linn Majik DSM is a superb piece of audio engineering, simple to use and sounds totally amazing.
Worth all of the £ 4965 it costs, and better than systems from other companies that cost more.
Music from a hard drive, itunes library, Spotify etc in conjunction with local sources from tv/sky/blu ray or even a turntable.
This ticks all the boxes, who needs surround when two speakers sound this good. Ask for a listen and you won't be disappointed.
The extra three rooms are costed as kef ceiling speakers driven by either Linn or Sonos amplifiers allowing you to play different music in different rooms simultaneously.
The difference in cost is because the Linn sounds better.