Is it worth to bother with iPhone 4 when developing a new application?
You're going to run into trouble if you try to make your application code device-specific. The correct way to approach a software release is to target OS-compatibility. That means that if you're going to go for a minimum of iOS 7, you need to ensure that your software is compatible with all the devices that are able to run that iOS version. Anything else opens you up to a whole world of hurt. App Store does NOT define system requirements by device model.