Hi Asem,

From your post I see that your iPhone 4 is unexpectedly shutting down while you're using it.  To help resolve this issue please follow the directions outlined below.

If your iOS device restarts, displays the Apple logo, or powers off while you're using it

You might see this unexpected behavior on your iPhone, iPad, or iPod touch if you have an older version of iOS. These steps can help. 

 

Make sure that your device is up to date and your apps are up to date. If you still see this issue after updating, contact Apple Support.

Learn what to do if your device won't start up past Apple logo or doesn't respond.

 

If your iOS device restarts, displays the Apple logo, or powers off while you're using it - Apple Support

Have a good one,

Alex H.