Do you like your house, car or other place where you charge to be a fire free, non-scorched zone?
All bets are off when you charge that battery if you even can. Good battery chargers make you plug into the balance ports and detect each cells voltage so they will not charge that battery. Bad battery chargers ignore the balance port and just see the series battery. Even those might say the voltage is too low. What also happens in LiPo batteries is the low cell just becomes a dead short. OK, in and of itself bad, but when you go to charge with a series (non-smart balancing) charger that just looks at the series of cells and you tell it 4 cells but you are charging 3 because one is dead, the other cells see higher voltage and then burst into flames. Murphy says this happens right after you walk away from the charger thinking all is OK.
That said, I had a pack like this. What I first attempted to do was charge the bad cell at a very low rate via the balance port using this http://adafruit.com/products/259 single cell USB charger. The cell died as a dead short later and my only choice was to make the pack a 2 cell (mine was a 3s) by removing the bad cell. I did this "test" charge attempt under supervision in a fire proof container. I still regret it because it was a waste of my time (the cell was dead anyway) and very dangerous.
Find a cheap place to buy LiPos, like Hobby King. That way it's less painful to throw away bad ones. A new battery is a lot cheaper than a fire in your house/car.
I gave first aid to the battery by giving 15v so long time that the battery voltage rose over 12v. Then the LiPo charger accepted to charge it again. The battery seems to work normally???
The problem here is that battery chemistry is unique from one manufacturer to the next. Quality of components is also a factor. Your batteries may have been on the fine limit of destruction and you narrowly escaped. The low voltage limits are based on thousands of tests on different batteries and compiling all that data. What is known is that any such event certainly affects the overall number of charge cycles and lifespan of the pack. One event could reduce the total number of charges by 1/2 or more. You also took the extremly dangerous aproach of applying voltage to the series pack, rather than controlling voltage to each cell one at a time. As I stated, all it takes is for a cell to not accept the incoming voltage and thus other cells see an over voltage as it must go somewhere. I wouldn't depend on an external balancer as things could still get out of hand. This nearly aways results in fire. There are hundreds of videos all over the net showing how little it takes to make one burst into flames just by exceeding the voltage even slightly. I cannot say what you did is OK even though you appear to have had success. This also doesn't mean on the next flight, you won't have a failure in flight. It's highly likely this pack will puff under high load or just normal charge discharge cycles.
The correct way is to charge each cell individually using a LiPo charger rated for one cell at a very low rate and monitor constantly in a fire safe setup. You greatly reduce the risk of causing a fire as it's nearly impossible to exceed the cells voltage causing the breakdown. In addition, The charge can make a good test of each cell and possibly give you insight into the relative damage done to the cell based on how much charge it takes in.
In my own experience with LiPos, I left one on a charger and the charger came unplugged. The charger discharged the battery over a few days and one cell puffed, the other didn't. I kept the "good" cell thinking I could just make another battery pack. Months later It failed the first time I actually threw some load at it.
Lesson here is that you now cannot "trust" that pack.
I'm very aware of the risks that charging under voltage lipo-batteries may cause. I charged the battery under surveillance and the battery was inside a metal box during charging. I just charged the battery just enough to get the minimum voltage to the cells so that the lipo-charger could take over. Lipo chargers usually doesn't start charging if the cell voltage in one of the cells is too low. I also store my batteries in a safety box.....
I'm glad you took the extra precautions.
Everything should be fine by the sound of it. I would take the time to cycle the battery a couple of times on the charger before doing any high amp currents. And remember to check if the cells still are reasonably balanced after normal usage, if they are and there is no puffing then the battery is fine.