Is my iPhone four useless?

Lexi, rice is great for cooking but it will not stop corrosion nor will it fix your phone. It is the equivalent of doing nothing. Since it did have an immersion the first thing I suggest doing is to not longer try to turn it on or charge it. It is easily possible that you are shortening out parts on the logic board i.e. the power management IC. If anything, disassemble your phone using these guides. Once you have that done, clean your phone with high grade isopropyl alcohol and this guide. It was written for a 3g but all the points are valid for your phone as well. Once its clean, and you replace the battery, reassemble and re-evaluate for further (if any) damage. Hope this helps, good luck.

I have had so much experience with wet phones. Once you have done what previous comment says, I would definitely invest in an otter box or similar case!!! Has saved me more than a few times!