Just a shot in the dark here...
It is possible that you have a cab that was lazily designed, or re-designed.
to get to 4 ohms with six drivers it either takes special drivers not found off the shelf, or seat of the pants engineering.
It's possible that the bottom 4 are an 8 ohm load together, as is the top 2.
For example someone could series/ parrallel 8 ohm drivers in the 4x10 and series wire a pair of 4 ohm drivers for the 2x10 portion. This would result in a 4 ohm cab with off the self drivers, and would cause those two drivers to piston far more then the other 4.
I can't remember off hand, but i seem to remember talk that one major manufacturer did this in the 80's and 90s.