For reference, I should quote Paul Scarpelli, who posted his answer to why people design four ohm speakers -
If you look at specifications of raw drivers from the driver manufacturers, you'll sometimes notice 4 ohm and 8 ohm versions of the same units. The 4 ohm versions are usually more sensitive, and not by a little. A speaker system that's 4 ohms will usually cause a good amplifier to deliver more output than it would into an 8 ohm load, too. The net pickup from these two observations can be 3-5 dB more output. More output means less distortion, more headroom, and the capability of playing substantially louder.
I will try to explain what I think he means, but I could be wrong. A 4 ohm speaker will require less voltage to drive it. If you were voltage limited, that would make sense.
Amps designs trade off voltage and current in their design. You can raise rail voltage which you might think would help reduce the chance of clipping. But you can't do this with no tradeoff. With a higher voltage, the power supply (assuming unregulated) will likely collapse under load. Of course as you started with a higher voltage, you have some room to work with.
On the other hand, you can do what I think Harmon Kardon does. You start with a lower supply voltage to the amp. In return, you should get better performance into lower impedance loads. But you start with a lower supply voltage, so even though it should be more stable, the clipping point is lower.
If you knew for a fact someone was going to hook up a well behaved 8 ohm speaker to your amp/receiver, you would simply design for that. If you knew someone was going to hook up a well behaved 4 ohm speaker to your amp/receiver, you would use a lower rail voltage than our theoretical 8 ohm scenario, and your current capability should be better.
(This is probably a simplification, and maybe has some inaccuraries, as I am not an engineer.)
But anyway, in reality, you don't know exactly what people will hook up. So you probably design for a compromise. And you put in limiter circuits and thermal shut off circuits to protect your product from crazy people
Not sure what my point was, now that I think about it. Sorry. Been a long week.