Then there maybe an Ohm setting on the amp to switch between 4 & 8, highly doubt there is a 16 as it's not widely commonly used. Or most likely, the amp is running in High setting (8ohm) which can also allow the use of 4ohm speakers to be used without over driving the amp creating excessive heat.
Found this information after I spoke lol.
Some amps have an impedance selector switch. In most cases we recommend the 8-ohm or more setting. The manufacturer puts them there for UL/CSA approvals as well as easing consumer concerns about driving low impedance loads. These switches step down voltage feed to the power sections which will limit dynamics and overall fidelity. Keep the switch set for 8 ohms regardless of the impedance of your speakers
The "low" setting (less than 8 ohms) and the "High" setting (8 ohms or more). This is the reason I usually recommend keeping this switch in its default "High" setting and using common sense when mating a receiver with inefficient 4 ohm speakers in large rooms.
All the "Low" setting of the switch is doing is stepping the rail voltage down so when UL tests the amp at a specified distortion level, the amp will achieve that distortion level sooner since it runs out of headroom more quickly than it would in the "High" (8 ohm or more) setting. This in turn generates less heat since the amp isn't driven as hard. You really aren't buying any protection for driving low impedance loads as you actually risk clipping the amplifier more since it can run out of headroom more easily. The switch is there more for certification purposes.
The reason you don't see this switch on separate amps is twofold:
They typically have more heat sink area, and bigger power supplies and can better manage the heat
They aren't UL certified and don't have to meet the requirement.