Apple has released its iPhone Software Development Kit for iOS 4 to all iPhone developers. Previously available only to paid developers, the updated SDK allows all registered developers a chance to develop OS 4-compatible apps, although the $99 annual fee will still be required for on-device testing and submission to the App Store. Notably, at least some of the accompanying documentation make references to iPhone OS 4, which was officially renamed iOS 4 two weeks ago. TUAW notes that Apple is expected to rescind its prior NDA covering iOS 4 developers now that the SDK and software update are more widely available. The iPhone SDK 4 s available now as a free download for all registered iPhone developers from the iPhone Dev Center.