Peg, a speaker, regardless of its impedance, draws the amount of power from the amplifier that it needs to put out a certain sound level. So, the receiver doesn't have to be "smart", it's the speaker which decides how much power it needs. Assuming that the receiver won't shut down if some of the speakers have a 4 ohm impedance(generally it'll be okay)the fact that the speakers differ in impedance is of no significance.

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Enjoy the music, not the equipment.