I was under the impression that Lower impedance makes a speaker harder for an amplifier to drive (because it requires more voltage), but also enables the speaker to play louder with that extra voltage.
If you think of the water and pipe analogy, a lower impedance speaker is a larger diameter pipe, which of course allows more water (current) to flow, but at the same time requires more pressure (voltage) from the amplifier to do so. But of course the impedance of any speaker changes throughout the frequency range, and many speaker companies publish an impedance chart for their speakers.
I've never seen any proof however, that a 4-ohm speaker is going to be "Better" sounding than an 8-ohm speaker, or vice-versa.