DP, as some of the previous replies indicated, despite the language in the manual, never switch the receiver to the lower impedance number. This language is legally required because of UL(Underwriters Laboratories)testing for overheating and fire prevention. To minimize the possibility of the receiver overheating with a lower impedance load, setting the receiver to a lower impedance number reduces the voltage output of the receiver's power supply section. Following Ohm's Law, when the voltage is reduced so is the current and power. So the chance of overheating is reduced because of the lower current, but at the expense of reducing the designed maximum power capability of the receiver. Tests have indicated that it's cut roughly in half.
As to the impedance spec on your M80s, it's possible that at one time the spec was based on an average impedance(for which 6 ohms would be a reasonable number)rather than a minimum 4 ohm impedance. In any case, it's nothing to be concerned about, and the setting should remain at the higher default number.
Enjoy the music, not the equipment.