In a similar situation to a previous story, I was a dolt and left my iPhone 4 in my swim shorts pocket and jumped into the pool to help my 2 year old get over the fear of swimming. I did not realize until I got out of the water about 10 minutes later that it was in my pocket. The phone seemed completely DOA. My sister suggested that I put it in rice, and I had nothing to lose. I took the back cover off, dried it as much as possible by hand with a paper towel, and left it in a bowl of minute rice for about 3 days. After a few days, the phone still seemed dead at first, as the power button did nothing. Still having nothing (else) to lose, I plugged the phone into wall power, it started to turn on, and was actually operational. I thought I was out of the woods, but shortly after unplugging the phone, it would just randomly shut down unless it was connected to a power source. I have a few travel external backup batteries that I was prepared to use until the new phone comes out this year. I read a bit online (including the similar account in this forum, and the video replacement guide), and decided that a new battery might do the trick. For the price of a new battery on this site, it was worth a shot since the repair was a quick operation.
Even though I am in the IT profession, the repair process was absolutely one of the easiest hardware fixes I have ever done. If you need a replacement iPhone 4 battery for any reason, you can surely do this swap, and you do not need to be afraid if you are not used to fixing electronics.
There is really nothing much to say for advice, other than just follow the video guide provided on this site. I did have the stickiness issue that others reported when trying to lift the battery out by its tab, so just be careful when prying it up. I am so happy that I have come out of this incident with basically no repercussion after such a stupid mistake on my part. I even had a bit of screen damage from the water that has 95% fixed itself somehow. I may possibly be even better off than before the accident with a new battery in a two year old phone.