You can jailbreak your iPhon4 and it won't ruin your phone, in-case anything wrong happens, you can restore it to factory settings and your phone will be as good as a new one. To jailbreak an iPhone4, you need JailbreakMe 3.0 version. You need to back up all your data and information and follow the instructions of using JailbreakMe software. For more information check it out on www.en.wikipedia.org/wiki/IOS_jailbreaking.
You can (Pretty sure) restore it back to factory settings, but Jailbreaking requires you to break the phone's warrinty, so, even if you do jailbreak your iPhone and put it back to factory settings, it will not be an Apple product.