iPhone 4s shutdown problem
that's quite simple!! That means you need a new battery...

The % indication is not always most accurate, specially on older batteries.

....when your iPhone automatically shut down yet the usage indicates that you still have about 8 or 11 percent. that means your battery is dead or almost,not to mention your iPhone shut down while it indicated it still have about 20 or 30 percent!!! 

I had that experience, I just replace my iPhone 4s battery these days since I found the same problem happened on my phone, it automatically shut down yet it says it still has about 11 percent of battery