This tutorial explains how to enable the Dual Window feature of the LG G4 smartphone. As the name already suggest Dual Window allows you to have two apps running side by side on the screen of your device.
So here's how to take advantage of the large screen of the LG G4 and at the same time boost your multitasking capabilities:
Go to Settings > General and toggle Dual Window on. You can also tick the Auto-open checkbox if you want your smartphone to open links or email attachments in dual window mode.
Dual window only works for a few selected apps (most of the stock apps provided with the smartphone). To use this feature, simply tap on the Recent soft key (square icon) and then on Dual Window button displayed at the bottom right corner of the Recent Apps (multitasking) screen. All the compatible apps will be displayed in this section (swipe left to display more apps), select drag the apps you want to use (one at the top and the other one at bottom of your screen) or tap on Recent to open the two most recently used apps in dual window mode:
NB: There exist a similar feature on the Samsung Galaxy S4 (and later models) called Multi Window.