My iPhone 4 Battery is draining fast, Why?

My iPhone 4's battery has been draining very fast for the past 5 days. It will go from a full charge (100%) to dead (0%) in about 4-5 hours, where as before it would last what seemed like forever. I have done the following…

-close all running apps
-deleted all useless apps that I was not using
-not used wi-fi
-dimmed the back light

The only thing that seems to be different since I noticed the draining power, is I have downloaded 'App of the Day - Free' and it gives you free apps that are only free for the day. I have since removed that app and all the 'free' ones that I downloaded. I also had downloaded the app 'Find my iPhone' I did some research and I found out that that one can drain the battery so I removed it. It has seemed to have not helped. Can an iPhone get a virus from an app?

I have reset all my settings to how it came and that as well hasn't helped. Any ideas? I'm still on contract for a year and a half and did not get any extended warranty. If I replace the battery, what are the costs, I live in Canada.

Thank you for your time, and I will appreciate any answer that can help.

Added (1). I'm also wondering if anyone else had had these issues since the update to 6.0?

Had this issue with an iPhone before. It would feel warmer than normal and the battery drained fast. Try backing everything up on a computer using iTunes, then use iTunes to Restore. It will wipe it back to original factory settings. You can then load everything back in from the back up. After that mine worked for fine for more than a year. If that doesn't help, go to an Apple store or call Apple for a possible warranty exchange. I assume the process is the same in Canada. In the US, batteries are about $80-$90 if not under warranty.

I have this phone since february, and starting to have the same problem. Hope somebody can give us some better explanations.

Very, very sorry for giving a thumb down! It was a mistake! This phone it is too small
to use it as a computer.
Thank you very much for taking the time to give us the in formations!

Have the same problem with my iPhone 4S.

Mine went from 100% at 7AM to 68% at 9AM - and it was off for about an hour. What gives?

I had my phone on charge all night when I woke up at 7 I took it off and went to have a shower, I didn't not use it to play music or anything yet when I got out of the shower 10 minutes later it was on 93%? This is really frustrating since I know when I go out to school tomorrow its going to die by lunch. Someone please help me to try and keep my battery life