First, not all of the apps on your iPhone 4 stay open when you close them. When we talk about how you close apps running in the background, you will see that all apps that you have closed normally, using the Home button, might look like they are running in the background, or multitasking. That is not true now, nor is is likely to be true in the future. First, the app’s programmers need to recode each app to take advantage of multitasking. Second, Apple will need to approve those apps which are eligible to run in the background, something that they may not do with all apps.
Nevertheless, the iPhone 4 and 3GS with iOS4 keep a list of recently opened applications, including those that take advantage of multitasking. In order to access that list, you need to double-tap the iPhone’s Home button, which may be a bit of a weird concept to many. Simply push the button twice in rapid succession, like you would with the mouse on a double-click.
When you do so, the icons in the bottom row will change, showing you the last four apps that you had open. Flicking the bottom row to the left will show more icons if more than four programs have been closed recently or are running in the background. This is, by the way, a quick way to reopen a program that has been recently closed or to bring a multitasking app to the foreground. Just tap an app in this bottom row of recent apps and it will run.
In order to completely close any background app, or remove an item from the recently closed “list” press and hold any icon on the bottom row. After the icons begin to wiggle, and each shows a red circle with a minus sign in the upper left corner, just tap on the minus sight for each app that you want to fully close / remove. When you’re done closing / removing items, pressing the Home button once will close the list removal mode and show the recently closed / still open icons. A second push of the home button will restore the bottom row to its normal state.