... is a speaker with sensitivity of 100 at 8 ohms as loud per watt as a speaker of sensitivity 85 at 4 ohms?
If the specs are accurate (and speaker sensitivity and efficiency specs are often optimistic by a few db), and if the specs are defined on a per watt basis, an 85 db 8 ohm speaker will be equally as loud as an 85 db 4 ohm speaker, for a given input power. A 100 db 8 ohm speaker would be **vastly** louder than both of them, for a given input power.
However, such specs are most often defined on the basis of an input of 2.83 volts, rather than 1 watt.
2.83 volts into 8 ohms corresponds to 1 watt; 2.83 volts into 4 ohms corresponds to 2 watts. 2 watts is 3 db greater than 1 watt. Therefore an 85 db/2.83 volt/1 meter 4 ohm speaker corresponds to only 82 db/1 watt/1 meter, while an 85 db/2.83 volt/1 meter 8 ohm speaker corresponds to 85 db/1 watt/1 meter.
In that situation, **if** the amplifier is capable of providing twice as much power into 4 ohms as into 8 ohms the maximum volume it will be able to produce with the 4 ohm 85 db/2.83 volt speakers will be exactly the same as the maximum volume it can produce with the 8 ohm 85 db/2.83 volt speakers. Again, assuming the specs are accurate.
Second, you may find it informative to plug some numbers into this calculator: https://myhometheater.homestead.com/splcalculator.html
You’ll find that under conservative assumptions about placement 30 watts into a pair of 85 db/1 watt/1 meter speakers will result in a sound pressure level of 93 db at a distance of 10 feet. Less conservative assumptions will increase that volume by an additional few db. And doubling that power to 60 watts will only result in a 3 db increase.
Given your preference for "lower listening volumes up to moderate," and the fact that you are in an apartment, I would expect that you would exceed 93 db at the listening position very rarely, and then only on very brief and very occasional dynamic peaks on certain recordings. Consequently, when and if that were to happen a correspondingly brief excursion into class AB seems to me to be almost certain to be unnoticeable.
Given all of that I would strongly recommend that you put aside your stated preference for a 4 ohm speaker. And as you realize doing so would greatly expand the range of available choices.
Also, you may find this paper by Nelson Pass to be of interest, if you haven’t already seen it: https://www.passlabs.com/press/leaving-class
Good luck. Regards,