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.
Sources:
Jailbreak 4.1
Jailbreak 4.2.1
4.2.1 Jailbreak