The "4 ohm option" should never be used. It reduces the voltage output of the receiver's power supply section, with a resulting reduction in current and power because of Ohm's Law. This is done as a safety measure to reduce the chance of overheating with low impedance loads, but does so at the expense of maximum performance(lab tests show that maximum power is cut roughly in half). So, the result is directly contrary to "optimizing" the receiver for 4 ohm speakers, which some imagine to be the purpose.
