One of Samsung's claims to fame is a feature meant to improve productivity on mobile devices. One that users of stock Android and manufacturer skins alike have been yearning for for a while. That feature is multi-window, which allows users to run two apps on the screen at once, dragging and dropping between the two.
The problem is no one has been able to get it right yet. A company in the mobile space - in this writer's opinion - has yet to perfect the balance between utility and intuition when it comes to multi-window functionality on tablets (or phones, though I haven't used the Note 4 yet), but a post to Android Internals in March confirmed that Google had been working on the programmatic side of multi-window in stock Android.