This is bar-none the easiest repair you will ever perform on an iPhone. Take the opportunity to save some cash and build some DIY street-cred by rocking this repair all on your lonesome.
Purchase an iPhone 4 rear panel in black OR white, here: http://bit.ly/iphone4rearpanel
Be a good little fixer and follow the instructions, here:
Outfit yourself with the proper tools, here: ifixit.com/Tools
And liberate your iPhone from its pentalobular oppressors, here: http://bit.ly/iphone4lib