Limera1n is actually one of the best programs that you can use to jailbreak your iPhone 4. All you have to do is go to the Limera1n website to do so. You can go here for the download: http://www.limera1n.com/. Ultrasn0w is another good jailbreak program to use for the iPhone 4. You can go here for that one: http://blog.iphone-dev.org/.