Although I always have the greatest respect for opinions expressed by Ralph (Atmasphere) and Grant (Tvad), and I almost always find myself in agreement with both of them, in this case I think that their posts are commingling two separate issues.
ANY amp having negligibly small output impedance, such as most solid state amps, WILL double the amount of power that is delivered into a halved load impedance, PROVIDED that it is operated within the limits of its maximum voltage, current, power, and thermal capabilities. That is a matter of basic electrical engineering principles. If in fact Harley disagrees with that he is wrong.
Most, and perhaps nearly all, tube amps will not do that, BECAUSE their output impedance is not negligibly small.
All of that is essentially a separate issue from the one I believe Emerson (Foster_9) was asking about, which I believe concerns whether or not the MAXIMUM power ratings of a solid state amp are twice as much for a 4 ohm load as compared to an 8 ohm load.
03-15-11: Tmsorosk
...there may be other reason's it doesn't sound bright other than just the double down aspect.
I suspect this is correct.
Best regards,
-- Al