Most modern AV Amps can drive simple Home Theatre speakers in an apartment, even if it says 4 ohms on the box. Especially if there is a subwoofer to handle the energy sapping low frequencies.
But for big, loud and dynamic moments, and if you switch out from those budget speakers to more demanding designs, such as Dynaudio speakers, then the amp will shut down.
A lower impedance speaker i.e. 4 compared to 8 ohms is easier to drive. This is a fact. Less resistance, more current flow.
MOST AMPS ARE NOT EQUIPPED TO HANDLE 4ohms.
Why? Because of heat issues. In theory all the energy is channeled to watts, and used to generate sound, but in reality, no amp is that efficient, and especially for Class A amps, it becomes heat energy. This kills amps. So most modern AV Amps, which have poor heat sinks - metal costs $$ and that's where companies scrimp on their designs. As a result, when one switches to the "4 ohm" setting, actually what happens is that the amp throttles the current, REDUCING, not allowing the full current to flow.
This preserves the amp and prevents a meltdown.
So why don't all amps melt down? Well, remember, most times, we Never hit the stipulated power, i/e most times, we only use around 2-10amperes of currrent. Even at 10A, that's plenty. Only a few designs hit 20A (Harmon Kardon and NAD spring to mind amongst the budget designs), and only for seconds.
In a really solid design, with THX Ultra for example, the amps are built with sufficient cooling to handle such heat. Most good amps do not need this switch. But the litmus test is whether they can handle Electrostatic Speakers, which often dip below 2 ohms.
For example, see the heat sinks on this giant hybrid tube amp:
A link to THX certification:
I have no financial interest or other interests in any of the items / events I write about.