Like I said, it's not likely a full discharge would kill the battery entirely after only 3 or four times.
As I have mentioned before I was in the industrial battery business.
What probably happened was that the battery was so discharged the chemical reaction ran to completion and the voltage dropped close to zero.
What happens when the voltage is very low is that automatic chargers will not detect a battery's presence and will not even turn on...making at appear that the battery is completely useless.
I've seen them so low that we've had to dial up the voltage quite high (sometimes 36 or more volts) and carefully monitor the current. As soon as the battery starts accepting current, then the voltage can be turned down until the battery normalizes.
What happens when a battery dies is that the plates simply crumble away. With your standard lead-antimony battery you have the ability to cycle it many times. I've never seen a lead-calcium battery destroyed by only
3 or 4 full discharges, but I have seen plenty of batteries with the voltage so low, the charger would not start.BTW: A lot of the old timers said that those ancient vacuum tube chargers were much better than their modern day equivalents. I have a few of them in my "museum" and they were designed for charging six (or possibly more) 6 volt batteries in series.
Since the output could be cranked up over 36 volts, one of those chargers could easily recover a "zero" voltage battery.
One more thing. If a battery is left in a discharged state, stage three sulfation will begin to occur. It may take months for the battery to fully crystallize, but once it does, there is nothing that can bring it back. That's why batteries should always be stored fully charged and in a cool place.