How to take the battery out of an iPhone 4?

I was wondering how because i need to do it for water damage

You have to buy a special kit. You can't just replace an iPhone battery like a normal cell phone battery. You have to take it apart and take off the glass plating. Do a google search for "iPhone 4 battery kit" and you will find what you need.

IPhone batteries aren't designed to be user-replaceable; that's just one of the compromises that iPhone owners make for choosing an iPhone over a BB or another phone w/a user replaceable battery.