If you purchased an iPhone 4 before September 30, 2010, follow these steps to receive an iPhone 4 Bumper or a select third-party case from Apple at no charge.
Download the iPhone 4 Case Program app from the App Store.
Launch the app on your iPhone 4 and sign in using your iTunes Store account or Apple ID.
Select your Bumper or case.
For iPhone 4 purchases made before July 23, 2010, you must apply no later than August 22, 2010; otherwise, you must apply within 30 days of your iPhone 4 purchase. To qualify for this program, you must purchase your iPhone 4 by September 30, 2010.