Sensitivity

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.