How to jailbreak iPhone four
It's not a good idea that you jailbreak your iPhone it'll mess it up
wouldn't recommend it, because even though you have a warranty and it's jailbroken they will not both fixing/repairing.
Why do you want to? What iOS are you running? To find that out, go to settings, general, about.
My brother has jailbroken my Ipod 4, it didn't get messed up. You have to download software to your computer and plug in your Ipod/Iphone. I'm not sure what the software name is.