Ohms are a measure of resistance and as such the lower the ohm rating the more flow of current to the speaker, by matching the nominal impedance of an amp it can deliver its full power to the load, when you look at the specs of any amp the watt rating increases as the load (ohm) decreases. The lighter load of 8 ohm(on a 4 ohm nominal amp) requires the amp to deliver less current and does not stress the amp as much as it is not able to deliver it's full power.
So, if the box used 8 ohm drivers the output would drop and not as you suggested increase. If the amp was 8 ohm nominal output the power requirement of the box would increase and battery life would suffer. The reason for low load is to maintain a level of efficiency from the small amp sections and battery power of boxes.
Now you might have noticed I have used the words "nominal impedance" this is because the load of any driver varies with frequency response. A sub as an example might be 4 ohm nominal but drop to 1 ohm (or lower) during certain frequency reproduction, a 8 ohm nominal home speaker might vary from 4 - 20 ohm depending on the frequency.
1000 % this man know his stuff.