iPhone 4 goes to black in the middle of viewing an app
Why does my iPhone go to black while I'm viewing content? Could this be related to the battery?
iPhone 4
Is it always the same app? If so, you know what the problem is. Otherwise, perform this procedure:
Close all open apps by double-tapping the home button, then swiping up and off the screen with the app window (not the smaller icon).
Reset your device: hold down the home button along with the sleep/wake button until the screen goes black and you see the Apple, then let go. (No data loss)