As mentioned above, saving a screenshot is ridiculously easy on Android devices, and the same can be By all LG phones. All you need to do is simply push and hold both the power button and volume keys while idle. Since the G4 has buttons on the back may feel strange, but place your finger in a way in which you can press both at the same time.
Briefly holding down the power and volume down buttons simultaneously and release it, and see the animation on the screen and listen to the capture screen. That is all. Your phone saves everything that is being displayed, and can be accessed from the gallery. You can do it as you wish, or enter edit mode and trim the edges while adding a little flair to any of the many editing tools available for Android.
If you try and it does not work, try again. They are hit simultaneously with a brief pause, and then let go. Timing is everything, and it's easier than you realize.
But there is a second method that is technically even easier, but still find myself using the above method of habit. LG has a feature called QuickMemo + is essentially a way to quickly and easily take notes or reminders, and screenshots. QuickMemo Press the + button on the quick adjustments drop-down notification bar and screenshots what is on the screen. From here you can draw on it, to edit, crop and more similar to what we have mentioned above.
Everything is easy and only takes seconds. Personally button combination is easier, because you can then only share from your gallery or bar notification, versus having to keep and do all the other things with QuickMemo +. The LG G4 will be released from most companies in the US soon, and we'll have more about how tos as liberation.