Because the times of impedance matching by transformers like in the tube "age" are mostly considered "past", modern solid state amps don't have that problem any more.
This generation is rather limited by heat and some regulations. Thus a 4-Ohm setting will just introduce some limiting algorithms into the equation to keep output currents for low impedance loads in check, limiting current and heat dissipation.
Switching to a 4-Ohm setup will usually limit or even reduce the stated output power of the amp.
As long, as you don't drive that amp continuously in high power mode and thus increasing the heat generated (because of high currents into low impedance loads) beyond the heat dissipation capabilities of the heat sinks and fans provides, it will be better, to let that solid state amp stay at its 8-ohm setting.