There are two methods two achieve this,
Using Hard buttons
I believe that the above image is self explanatory. Just press and hold the Home and Power/Lock button at the same time, it is as simple as that.
Using incredible swipe screen feature
This is the simplest method, just swipe your hand over the Samsung Galaxy S4 as shown in the above image, oh boy! that’s it your screenshot is cooked and ready to serve.
Now go on and take screenshots of your Samsung Galaxy S4 where ever and when ever you desire.