To jailbreak iphone 4 allows you access to more features which are otherwise inaccessible to you. Although this gives you more functionalities Apple has issued a statement that indicates that jailbreaking an iphone 4 makes the warranty void. This means that your iphone will no longer be in conformance with the terms of the warranty hence its invalidity.
Anyway this has not and it will not stop iphone users from jailbreaking their phones. The users are in a position to jailbreak and restore their phones to their original states using iTunes, thus users can enjoy both worlds.