What's with 4 ohm speakers?

In tweeters the mass of the voice coil becomes an issue so I do not doubt that they would use 4 ohms over 8. All things being equal that would cut the voice coil weight in half.

Ribbons are often 8 ohms as the ribbon itself is the equivalent of a voice coil.

Speaking of tweeters, Morel offers nearly identical MDT29's in both 4 and 8 ohm versions. As rated, the 4 ohm is 92 dB sensitivity and the 8 ohm is 89 dB. For a MTM, the 4 ohm would be more likely.

So, just by halving impedance and not doubling driver area and motors, you get +3 dB.

I may be wrong but FWIU halving the impedance will not increase efficiency by 3db. If you look at the spec's more closely you will notice that Morel spec's the 8 ohm at 89 db at 1W/1M which is 2.83 volts into 8 ohms. If you look at the 4 ohm spec sheet they stayed with a 2.83v input which is actually 2 watts into 4 ohms. So the doubling of the wattage is where the 3db comes from.

It seems to be fairly common practice among driver manufacturers to spec a driver that is available in both 4 and 8 ohm versions in this manner. Can't say as I blame them since they believe that the majority of there drivers will be used on a "constant voltage paradigm" type of amp. Thus the amp is going to try to keep the voltage output the same regardless of impedance.