how to get back my "AAp store" application on Iphone 4....

The App store app is a built in app and cannot be deleted. Try:

1. Check settings to see if restrictions are turned on and/or if Installing apps is turned off (Settings app > General > Restrictions)
2. Check all of your home screen (by swiping left to right), and folders to see if was inserted into one. (you can also use Spotlight search which will tell you if it is in a folder. Swipe down on the Home screen and type in "app Store")