your sony does not require impedance matching. there's no output transformer to change the tap on, for example, like a tube amp. The four ohm setting simply limits power so the unit does not overheat when running tests with four ohm speakers. Leave it set to 8 ohms unless you find it's very very hot (which implies you listen pretty loudly or have inefficeint speakers and or a large room). Others may disagree but generally I'd trust the receiver's protection mechanisms to keep me out of trouble. The real problem with lower impedance speakers is they require the amp to deliver a lot of current when you get to high levels. If you listen 10 dB or more below reference, you're likely idling along at a watt or two except for the loud parts and nothing bad is going to happen.