Hi,

Prior to my remodel I had the same requirement as you. I tried out an Adcom speaker selector but decided to acquire an additional stereo amplifier to power the second pair of "Zone two" speakers (because more power is better, right?).

I can't help you with the theory or design of an impedance matching transformer to do what these types of speaker selectors do but I am sure there are people on this board who can...

As to whether showing a 4 ohm load to your amp is OK... that depends on the amplifier. ANY outlaw amplifier will do just fine with a 4 Ohm load. The downside of just wiring in parallel is that both pairs of speakers will always be on when you are playing zone 2. Using a speaker selector or second amp would allow you have the bathroom off while listening in the bedroom and vice versa.

Speaker Craft makes impedance matching speaker Selectors that have built in 'protection' that presumably will protect your electronics from damage.

http://www.speakercraft.com/#Products:159:Direct%20Connect%20S4dc

Regardless, you will still need to make sure your amp can handle a 4 Ohm load.

When planning our remodel I decided to go with whole house audio for ease of use and flexibility of distribution and have installed a Sonos system.
Two of the benefits of a system like Sonos, (or Slingbox, etc...) are that you can start small (one zone) and grow as you desire, plus they offer units with and without amplification.

Whichever way you go, you'll need to follow Gonk's advice on cabling.