Power button not working iphone 4s

All I could find was this

Buttons and Switches

Sleep/Wake button will not lock or unlock iPhone

1. To lock the phone, press the Sleep/Wake button. Note: By default, if you don't touch the screen for a minute, iPhone locks automatically.
2. To unlock iPhone, press the Home button or the Sleep/Wake button, then slide the slider.
3. If either of the above steps don't work, try turning iPhone off and then on again.
4. If the iPhone will not power off, reset it.
5. Try to lock or unlock the phone.

Not sure whether it really helps you but I mention it for what it is worth

I guess you will need to visit an apple store and have it assessed for repair

Is the phone under warranty? Check here https://selfsolve.apple.com/agreementWarrantyDynamic.do

If it is not under warranty they may do an out of warranty replacement but that is up to them. You have to check with apple