Always a contentious issue is this. A couple of points. Generally the impedance stated on the name plate of loudspeakers is a nominal value i.e. 4ohms or 8 ohms. Most speakers will have an impedance below this stated value at some point(s) over the audio band. No loudspeaker system has constant impedance over it's entire operating range. It's true that a pair of 8 ohm (nominal) speakers connected in parallel will create a (nominal) 4 ohm load to the amplifier, but this will in all probability be somewhere around 3 ohms at some point. The amplifier manufacturer will state an impedance limit for full load operation, usually with any tone controls set flat and this limit will be conservative. Yes, as stated above, you can run lower impedance loads on an amplifier, but on your own head be it. Although the impedance may seem O.K. and it may work, albeit at low volume levels, as the impedance seen by the amplifier output stage halves the heating effects on the transistor junctions increase by a factor of 4 exponentially. Remember that heat is the killer for electronics. So, if you have an amp rated at 8ohms minimum and decide to run 2ohm speakers, the impedance is 1/4 of that minimum, but the heating effect on the output transistors has gone up by a factor of 16. So although it may be possible to run as low as 2ohms the amp may not last long especially if you are inclined to turn the volume up.
By the way (I think some people get a little confused) having switched outputs for 2 pairs of speakers with both on at the same time is the same as connecting 2 loudspeakers in parallel to the same pair of terminals. Although you have separate switching for each pair of speakers you still have only one amplifier.
Hope this helps.
