How to close apps in iphone 4?
You have two options to close an app:


With the application hidden (i.e. if you are currently at the home screen or are interacting with another app) you can double-press the home button, hold down any icon in the app switcher until they all start to wiggle, and press the minus icon in the top left corner of the app you want to close.
With the application in the foreground, you can hold down the power key until the "slide to power off" option appears, and then hold down the home button until the the application quits.


The first option is faster.