4 ohms and 8 ohms

Solid state amps output a voltage which, within the power capability of the amp, is unaffected by the load. Maintaing the voltage with a low speaker impedance requires output of more current (amps), so the low impedance speaker will draw more power from the amp. It just happens... no switch required.

Tube amps make their output via a transformer which matches the tubes (which generate lots of voltage but little current) to the load impedance. A low impedance speaker will not draw more power from a tube amp. To get full rated power out of a tube amp you need to connect the speaker to the appropriate output terminals...4 ohms, 8 ohms, etc.

Cibr...

If an 8 ohm speaker is connected to the 4 ohm tap of the amp, the voltage delivered will be less than the amp could deliver at the 8 ohm tap, so the available power is reduced.

If a 4 ohm speaker is connected to the 8 ohm tap the current delivered will be insufficient for full rated power. (This is a bit like driving your car up a steep hill in high gear).

Either way, there will be no damage. Some folk claim that their 8 ohm speakers sound better on the 4 ohm tap. If you don't need full rated power this might be OK. However, the load presented to the output tubes via the transformer will not be what the designer intended. It will be too low (High impedance...confusing isn't it). I know that tube amps "don't like" operating into an open circuit (nothing connected) so perhaps this light loading may have adverse effects.