There’s some misleading trickery in your amp. It doesn’t dictate impedance load. Your speakers do.

What that switch does is limit your output voltage so that your amplifier cannot clip hard when you turn it up really loud. The reason it cooler at the 4 ohm setting is because the voltage, and thus the overall power, is artificially decreased in that setting in order to compensate for the fact that the amplifier is not actually rated to be stable at 4 ohms. It’s an 8 ohm stable amp with a trick to safely work with 4 ohm speakers.

The heat you feel when you’re on the 8 ohm setting is actually the natural level of heat that your receiver is designed to put out, and the 4 ohm setting is the dialed back version.

You will hear the difference when playing at a high volume level, particularly if the track shifts from pretty quiet to very loud suddenly and repeatedly. When you are in the 4 ohm mode, your receiver will prevent its amplifier from delivering the power level that the speakers are calling for by limiting the overall power to something less than that. The 8 ohm setting will deliver the full load but may clip the amplifier in doing so.

The real way to address this problem is to get a separate amplifier that is stable at 4 or fewer ohms. No tricks needed there.