The iPhone dock is a very useful feature allowing you to access your favorite four items on the phone at the touch of a button. The standard setup allows you to have four icons on the bottom of the home screen; however, if you feel the need for one more shortcut, it is possible to add a fifth button to this. This takes less than five minutes and anybody remotely competent at using an iPhone can do it.
Enter the iPhone application download center through the main menu.
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Search for "Five Icon Dock."
Select the application and click to confirm you want to install it.
Click and drag the extra icon you want into the taskbar at the bottom of the screen.
This also works for iPods.