To take a screenshot on Samsung Galaxy S4 you do not require to download any app to do this.
To take a screenshot just simply press and hold the home button and the power button together for about 1 seconds and it will take a snapshot of the screen. You will hear a camera picture taken sound.
The screenshot notification will appear on the notification bar at the top of your screen. The screen shots are available in your gallery.
Screenshot are saved in 500 x 899 pixels and in PNG format.