(no bulk, no equalize, just cut off at 41,5v). I think this is the accelerant of the problem.
While rapid charging is not very efficient, it doesn't allow time for the undercharged batteries to reach full charge if one cell is overcharged. If instead, stage 2 tickle charge or Slightly lower CV with declining current to near zero would allow the lower voltage charging cells to reach a higher potential. The weaker cells reach overvoltage sooner due to higher ESR, while the lower ESR cells which are better are at a lower voltage. Then when high load occurs, the overvoltage cells, with high ESR drop to a lower cell voltage probably due to 1 of four 1.5V cells.
I call it the weakest link reaction, where capacity and ESR are inversely related with aging and state of charge. Equalizing is faster with independant Voltage chargers, but still possible with 3 stage rapid charging with current sharing, where stage 2 prevents the weaker overvoltage cells from being aged faster with more current and the others to catch up to full charge with a slower rate of charge.
Unlike capacitors which would simply all integrate current into voltage and the weakest or lowest C value cap would charge up the fastest, batteries have non-linear properties below 10% and above 90% SoC. Below 10% ESR rises rapidly, and when over charged and overheats, ESR drops and overcharging accelerates.
This is a hand waving experience, I don't have data readily avail to back up, but is consistent with my experience. Hot batteries have more power but age much more rapidly. dead batteries have like worn caps low capacitance and high ESR.
You might have around 100k Farads in each battery, but the S.G. and ESR is what dictates aging. Smart chargers have temp. compensated 3 stage controls.