My iPhone 4s wifi don't work?

Take a look at this Apple Support article:

https://support.apple.com/en-us/HT204051

Reset your network settings.

Settings - General - Reset - Reset Network Settings

Reset your device. This won't delete your data stored on the phone.

• Press and hold the Sleep/Wake button
• Press and hold on the Home button
• Keep holding both buttons until the display turns off and back on with Apple logo on it.

Alternatively, you may go to Settings - General - Reset - Reset All Settings

https://support.apple.com/en-us/HT201559

Have you tried a 'hard reset' (this procedure is safe and won't remove any of your important data on the phone)? Restart holding down the 'on/off' button whilst simultaneously holding down the 'Home' button, until you see the Apple logo. Once booted, test to see if wifi now works.