Use your ears. It's easy to change on-the-fly. IMO there is a significant sound difference between the two (where there really should not be). The 4-ohm setting, when used even with 4-ohm speakers, sounds compressed with a near-mono soundstage. When you switch the setting to 8-ohm, the soundstage quadruples (at least) in width.

Stern is doing something else with this setting other than impedance matching with the drivers. I'd love to know what. I have a home hifi amplifier with a 4, 8 and 16ohm tap. Switching between them presents far less a sonic change than on a Stern machine.

In short - my advice is to keep it on 8. I'm yet to hear of anybody frying a driver due to this....