My iphone 4 battery drains randomly?

I make sure all of my apps and tabs and everything are closed. I've turned down the brightness. I've tried so many things but it still drains really fast sometimes. And other times it doesn't for some reason? What could be wrong?

You may be using your phone more when it drains fast, and less often when it does not drain as fast. If this is not the issue. It could be the charger or charging port that you are using to charge your phone. If you charge your phone using a computer, it will not charge as fast or as well than if you charge your phone using a wall outlet.

It could be your signal. If your phone isn't getting good signal, then your phone is constantly seeking for better signal so it is draining the battery. Either that or you left bluetooth on.