why my iPhone 4S auto shutdown when 25 battery remain

Maxime is correct, except that 4 hours is sufficient if you use the wall charger, or 8 hours if you use a computer USB port.

You must do this exaclty, your iphones battery meter is not calibrated with that battery. To resolve this issue you must:

-Discharge the device fully to 0%

-Recharge it fully,even if your phone says 100% do not unplug it (because its inncacurate) leave it in the  charger for at least 8 hours.

Doing this will recalibrate the battery capacity sensor in your iphone and fix the problem, this process is called a full charge cycle and should be done once a month for proper battery care and prolonged life

*Correction its at least 4 hours, thank you lawrence

I didnt do anything but it became normal again last night thanks