Hi, welcome to iMore.
This looks like a hardware issue to me. It's best you take your iPhone in to an AppleStore or a Apple certified repair service.
Also, the iPhone 4s has been on the market for at least 5 years, so you could maybe even consider getting a newer iPhone instead of spending that money on repair costs.
Good luck and if you have the time, signup for iMore to let us know what you decided. We're happy to have you.