battery of my iphone 4s decreases every 4 minutes

The usual first step with battery problems is to Restore iPhone Firmware with iTunes on computer. At end when prompted, Set Up as New iPhone. See if the battery performance is better after this. If still problems, Apple Store can perform quick (minutes) battery diagnostic testing. Also all iPhone 4S have Warranty, make Genius Reservation and take iPhone to Apple for resolution.

That would be 15% per hour and would last almost 7 hours.  Yes, that's pretty normal, depending on use.  See specified battery use here... http://www.apple.com/iphone/specs.html