bbbbbbbrrrrrrrIIIINNNNGGG!WOULD SOMEBODY PLEASE ANSWER THAT PHONE???
As mentioned, lower impedance should usually produce stronger bass, but that's dependent on a number of factors. In my case, I wouldn't be surprised if my system tested at 2 ohms to the big drivers under heavy load using the 8 ohm taps ... synthed bass at 11 kinda thang. So, as with everything, we compromise and aim for the best overall results across the audio spectrum, and let the crossovers sort it all out. Their sole purpose is to keep all the drivers happy with a proper load. Worst possible load will usually occur at the crossover points where their sonic impact is minimized.
(So yes ... drifting cap values could have a significant impact on what you're hearing.)
So, no ... I personally wouldn't use 4 ohm taps with an 8 ohm load, but whatever works for you. If anything, a finicky speaker might be best served with bi-amping.
And ya ... best way to experiment would be to make all the taps available for quick A/B comparisons.
Speaking of crossovers ... another project on the hit list as my XR16's are pushing 35 years old now.
Those bad boys will require a daughter card and additional bracing for support as they're a tad bit bigger than the OEM caps ...
Drivers have all been refurbed, so that should make the speakers like new. May also reseal the boxes as long as I'm in there ... not uncommon for them to leak some at the terminal plates.
PS ... speaking of drivers ... a LOT of the benefit you seem to be getting from the 4 ohm taps could be due to stiff action on the cones. My surrounds didn't look all that bad, but I had one of the big woofs go open, and figured as long as I was at it ... results were pretty spectacular when it came to defined bottom. Amazing what we can get used to over time.