You can get back the deleted Facebook icon on your iPhone 4 by redownloading it. Simply go to the "App Store" and tap on "Purchased" which is at the bottom of the screen. Then, tap on "Not on this iPhone" option, which will provide you with a list of apps that are not in the iPhone. Enter your Apple ID password and restore the deleted app. Go to http://lifeinlofi.com/2012/11/28/restore-deleted-apps-to-iphone/ for details.
Go on the app "app store" click on search on the bottom and then on the top type in "Facebook" let it load. Then a bunch of apps will pop up. Slide your finger until you find the app that looks like the one you had. Then click on where it says free. Then it will ask for your password for the App Store type in your password. And then it will load and then there's ur app ;)