Why does my iPhone 4S keep turning off?

My battery can b at 74% charged

The phone "sleeps" or darkens the screen to save the battery supply after about10 secondsbrightly lit.

It could be that the cable that connects the battery to the iphone can be missing its screw and could be loose so the connection can sometimes not go threw… I had this problem with an iphone 4 but i'm not sure if the 4s is the same because 4 has one screw that holds that part but the 4s has 2 wich can make it more stable.