Sensitivity

Usually, sensitivity is specified for a specific voltage from the amp, usually 2.83V RMS or 1W into 8 Ohm. The higher the sensitivity, the higher the SPL from the speaker for a specific amplifier output voltage. All else being equal, a 4 Ohm version of the speaker would have 3dB more sensitivity than the 8 Ohm speaker because for the same amplifier output, you putting double the power into the 4 Ohm speaker.

Sensitivity comes at the expense of enclosure size if you want to maintain a certain bass response. Higher sensitivity means bigger speakers. You need about 6 cubic feet for a vented enclosure for a woofer to reach 40Hz and maintain 94dB sensitivity. You only need .5 cubic feet for a smaller woofer to hit 40Hz, but you then only have 84dB sensitivity. You'd need to pump 10W into the 84dB/8Ohm speaker for every watt you pump into the 94dB speaker to hit 94dB.

Some ambiguity in the comments here! Let's see if we can straighten them out.

First- Sensitivity is indeed 2.83V at 1 meter. It is not the same as efficiency! Efficiency is 1 watt at 1 meter and there is a difference.

If the speaker is 8 ohms the two numbers are the same, as 2.83 volts driving 8 ohms means that there is 1 watt in the load. However if the impedance is reduced to 4 ohms, 2.83 volts is 2 watts.

What this means is that a 4 ohm speaker of the same sensitivity as an 8 ohm speaker is actually 3 db less efficient. Because the two measurements are often confused, the myth has arisen that if you put two speakers in parallel (for example two woofers) that their efficiency is doubled (3db) but this is not true. The sensitivity is doubled as the impedance is cut in half, but the efficiency remains the same.

Conversely if the same woofers were put in series, the efficiency is still the same as a single woofer, but now the sensitivity has been reduced by 3 db.

What is poorly understood is that efficiency means that there will be 1 watt in the system. So if 2 woofers are involved per our example, each woofer will dissipate 1/2 watt regardless of whether the speakers are in series or parallel.

Where this becomes important has to do with your amplifier. If tubes, the sensitivity rating can be misleading as the impedance plays a big role. Tube generally are less effective into lower impedances and are not known for doubling power as the impedance of the load is cut in half.

It might help to read this article- it explains why this is going on and why there are two specs:
http://www.atma-sphere.com/Resources/Paradigms_in_Amplifier_Design.php

As a further note something not discussed in high end audio but certainly should be is the fact that there really is no good justification for four ohms as a speaker load if sound quality is your goal. It does not matter what kind of amplifier you have! All transistor amps (while maybe quite comfortable driving 4 ohms) make more distortion into lower impedances.

You might think that the change in distortion is insignificant but its not. Our ears use odd ordered harmonics to sort out how loud a sound is; in essence any amplifier driving a lower impedance will sound brighter and harsher. Now if *sound pressure* is your goal then there is a weak argument for four ohms, as you might get 3 db more output out of the amp if its solid state. However you will find that the speaker cables are much more critical for four ohms.

It would appear to me that there is plenty of "... good justification for four ohms {sic} as a speaker load if sound quality is your goal." A four Ohm speaker has better chance of more sound pressure, better bass response, more linear impedance and ergo linear amplitude output frequency response, and likelihood of producing wave form fidelity than speakers with higher impedance's. This has been (and continues to be) borne out in the marketplace where thousands upon thousands of people have bought speakers with just such a load for just such a purpose. It just behooves one to purchase amplification that is up to the task.
Some cables might be better with the lower impedance load that 4 Ohms offers, rather than the higher impedance alternatives, especially when considering that it's easier to make a steady 4 Ohm speaker than a steady higher Ohm speaker. There doesn't seem to be shortage of available cable varieties to suit the need.

Unsound, I don't think you are looking at the physics, just the marketing.

Take a look at the distortion curve of any amplifier driving 4 ohms and compare to the 8 ohm distortion curve. Then factor in the fact that the ear uses odd ordered harmonics as a loudness cue, and finally that most of the distortion you are seeing in those curves is odd ordered.

There is a reason why some speakers are considered 'hard to drive'. It is because the amp has to work harder to drive it. What seems to be swept under the carpet is the fact that when an amp is working harder, its also not sounding its best.

...But (and IMO more importantly) the speaker is working more linearly.