What to do when Lenovo S850 switch off automatically?

It has been just a year taking lenovos850 model and the problem started occuring from February2017. My phone switch off automatically when used and without a charger or a powerbank i cannot use my phone for even a single hour and when it swich offs and tried to reopen a beep sound comes and just a white light flashes and dissapears in no time.When camera is used at a 90℅ battery the power offs without taking a picture and open when plugged to charger and appears 4℅ battery. I want my this problem to be solved soon please

I had a phone with similar problem. It just randomly shuts off. What I did was replace the battery and it never shut off by itself again.

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