It is easier to bleed charge from a single cell than to try and charge a single cell. The reason is because there is overcharge protection circuitry used in these packs and adding charge to a single cell messes with that circuitry.
What voltage level are you able to achieve on the trouble cell when attempting a balance charge with the B6 charger?
If you can get it to 3.2V, then bleed the other 3 cells down to that level and try balance charging again.
The pack is most likely toast, but it never hurts to try and recover it for ground operations.