I'd just leave the Onkyo on the 6 to 8 Ohm setting. TL;DR: the 4 Ohm switch is to pass UL/CSA certification and prove that an AVR won't catch on fire.

https://www.audioholics.com/audio-amplifier/impedance-selector-switch-1

I disagree on his other untenable stances (center channel comb filtering is a problem, 80 Hz as the ideal crossover frequency, etc.), but in this case I tend to agree with his assessment. And you'd be surprised at the number of audiophile components that don't have that those certifications.

Author of "Status 101: How To Keep Up In A World That Keeps Score While Buying Into Buying Less"