Whats the best/fastest/cheapest way to replace my iPhone 4's battery?

Will Apple store do that on a jail broken phone?

Not sure where you are located but my wife and I have both used http://rotten-apples.net/

You will be without the device for a couple of days, but if you have a backup phone you can put the SIM in, you should be fine.

Thanks.

If you go into an Apple Store, they will replace the iPhone with a refurbished unit or a new unit if they don't have a refurb in stock. They don't bother sending it away. I've seen this done for 2 different people. The replacement phone will have a 90 day warranty.

If you want your exact iPhone back then you have to mail it into Apple service and pay the additional shipping fee.

In addition to Apple, there are places that you can go to or mail your phone to that will do it for a bit less. I would check Yelp! and Google to see if there are any in your area that are recommended. MacRumors and Apple forums are good for info too.

I have no idea what they will do if Apple gets a jailbroken iPhone. I'd do some web research.

I used these guys twice and they were very reliable: rapidrepair.com

DIY is cheapest.

Iphone 4 battery replacement