To close apps on an iPhone 4S running iOS 7 or later, press the home button twice to display screen shots of all apps running in the background. Swipe from side to side until you find the screen shot of the app you want to quit.
Tap the screen shot, and swipe toward the top edge of the screen to close it. To close apps on an iPhone 4S running iOS 6, press the home button twice to display the background apps in the dock. Swipe the app icons in the dock from side to side to find the app you want to close. Tap and hold the icon, and when a red badge with a horizontal line appears, tap the badge to close the app.