Why does my blackberry lose power after 4 hours?

-Hi.Sounds like you have a defective battery to me.I would recommend trying a new battery in the phone to see if it works properly again.Several reasons might be the cause.

What are you doing with the phone during those 4 hours?



More info is needed, to give you a reasonable answer. But a few guesses are:



Not fully charging the phone will do this, using an older dying battery, not charging with a changer made for the blackberry.



Leaving 3G on when not in a 3G signal area. Leaving the blue tooth, GPS, and Wifi radios on, when not using them.



Leaving apps running in the background that connect to the internet (FaceBook comes to mind)



Streaming or playing music or videos.



There are also other reasons, but need more info from you as to what you are doing.