TimK1964- Batteries of that age certainly can go for no reason. However, as I mentioned before, the calibration algorithm 1) seems to be considerably slower on the 4S than on the 4, and 2) is critical for any battery that isn't in "like new" condition.
The charge IC contains a default charge/discharge gauge profile, and only through the calibration process does it adjust these parameters to reflect the "true" capacity at any one time, for your aging battery.
That the battery capacity seemed "abysmal" after the upgrade (when the battery gauge calibration was defaulted) I suspect it's the battery gauge and not so much the battery that has changed (although your battery failing at the same time as the upgrade is certainly possible, even if unlikely).
I would do as Pizza and sbaily suggest and do a DFU restore to default 6.1.2, setup as new (don’t restore from backup, etc), do the calibration procedure (literally run it for a day, then let it run down overnight).
Charge it fully the next morning. Then do it one more time., then use it normally for a few days, allowing the calibration fine-tuning to set, and see what the result is. If it's still abysmal, your battery may have in fact failed. The only other alternative, is the proposed theory that, because of the signal strength in your area, combined with some theorized bug in 6.1.2, is causing the radios to be over-used, causing battery exhaustion.
Take your pick on which one you believe. Unfortunately only time will tell.