What’s this switch for? It gives you the flexibility to change the impedance of the speakers so that you can utilize more power from your 4 ohm capable receiver or amplifier. If what you’re reading makes no sense to you, leave it on 8 ohms! Read on if curious.
This flexibility will also allow you to use speakers in a parallel/series combination to balance your system loads without an impedance matching device. See this post on a basic discussion on impedance here.
Another use could be to gain an extra 3 dB of output from your amplifier (if it’s 4 ohm capable), by setting them at 4 ohms. 3dB isn’t very much of an increase…think of it as an extra turn of your volume knob to the next higher power click.
So if you know what you’re doing, these speakers have flexibility in mind. If you don’t know, just leave them at 8 ohms and know that you’ve got it right.