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.
