FWIW a 'nominal impedance load' of 4 ohms doesn't preclude the load from going lower to say 2 ohms or less at some frequencies which would require a high current amp. Most folks I think stay away from tube amps for just that reason, i.e. they don't drive low impedance loads well. Using the 4 ohm taps doesn't remedy this much if at all. Best to ID your speakers actual impedance 'curve' before you try to match your amp and speakers. Asks the manufacturer (?) or look for reviews which might have identified it.
If an amp has 4 ohm taps
(and I am specifically referring to a tube amp) would there be any reason to avoid speakers with a listed nominal impedance of 4 ohms?
I realize this subject has been discussed in some form before, but I cannot seem to get a tight grasp on what makes a speaker an easy or hard load to drive. What I THINK I have gotten out of it so far is that the lowest impedance a speaker presents to the amp may be more critical than the listed sensitivity? (And also that for best performance the tap on the amp should match the nominal impedance of the speaker.)
So I would think that generally speaking, if one had an amp with only 8 ohm taps (which is probably includes quite a few amps) one shouldn't even consider speakers with 4 ohms listed as nominal impedance.
Generally speaking, is a 4 ohm nominal impedance a tougher load than an 8 ohm nominal impedance? But if the (tube) amp has 4 ohm taps, does this mean that using those taps it should be able to deal with a speaker with a nominal impedance of 4 ohms as well as it would deal with a 8 ohm speaker using the 8 ohm taps?
And here is another discussion that I found here on A'gon, but I am not sure if it answers my question:
https://forum.audiogon.com/discussions/amplifier-circuitry-4-ohm-vs-8-ohm