What's with 4 ohm speakers?

They're not hard to drive if you have a good amp. Also, 4 ohms is an average. Depending on the frequency of the music you are playing, the speakers resistance will vary.

Sensitivity.

Take a typical "8 ohm" midwoofer and double them up in parallel and you get 6 dB more. Realistically, more like 3.5 to 4 dB but also drops impedance by half. Problem is that impedance varies with frequency and the same midwoofer will have a minimum impedance, usually around 200 Hz. Some more than others.

The last drivers I used in a project had a minimum impedance of 5.6 ohms. In a MTM that's 2.8 ohms at a frequency that demands current. A 4 ohm tweeter doesn't have that current demand. Found a trick to eliminate that impedance dip but it doubles the cost.

Subwoofers are often 4 ohms because the trade-off for bass extension is sensitivity or size. So, they often use lower impedance to get more sensitivity from smaller drivers in smaller cabinets. Automotive subs are sometimes only 2 ohms. Same applies to woofers. Do you want bass? Do you want reasonably sized cabinets?

There's also the advantage of using smaller value inductors with lower impedance and less resulting phase shift.

They're not hard to drive if you have a good amp.

Depends on what you mean by good.

4 ohms is an average.

Not exactly; 4 ohm "nominal" has no commonly accepted definition. Plenty of speakers rated by manufacturer as 4 ohm nominal have very different impedance minima and maxima, which in turn will place very different demands on the amplifier that is driving it. Not to mention phase angle and sensitivity.

Depending on the frequency of the music you are playing, the speakers resistance will vary.

While true in the absolute sense, some speakers have a relatively flat impedance curve; i.e. they do not vary much w frequency. Others vary widely. And that's exactly the reason why amplifier-speaker matching is important and why there are many "good" amplifiers that will not mate well w a 4 ohm nominal speaker. And many good amplifiers that will not mate well w a 16 ohm nominal speaker.

I read a review of my 8 ohm speakers and it noted a drop to 3.something ohms here and there. Changed to the (tube) amp's 4 ohm output and huzzah...all better. Luckily there is plenty of info out there to make sure you don't mis-match yer stuff, although exceptions exist...i.e. high impedence/high efficiency things (Zu, etc.).