Reset iPhone 4S: Press and hold the On/Off Sleep/Wake button and the Home button at
the same time for at least ten seconds, until the Apple logo appears.
Restarting and Resetting iPhone 4s
If something isn’t working right, try restarting iPhone, force quitting an application,
or resetting iPhone.
Restart iPhone 4s: Press and hold the On/Off Sleep/Wake button until the red slider appears. Slide your finger across the slider to turn off iPhone. To turn iPhone back on, press and hold the On/Off Sleep/Wake button until the Apple logo appears.
If you can’t turn off iPhone or if the problem continues, you may need to reset iPhone.
A reset should be done only if turning iPhone off and on doesn’t resolve the problem.
Force quit an application: Press and hold the On/Off Sleep/Wake button on top of iPhone for a few seconds until a red slider appears, then press and hold the Home button until the application quits. On iPhone 3GS or later, you can also remove an application from the recents list to force it to quit.
Resetting iPhone via menu
Go to Settings > General > Reset and select "Erase All Content and Settings"