Unreal Engine 4 is the newest iteration of the Unreal Engine created by Epic Games. It provides an editor interface for building game worlds and placing actors, a content browsers for organizing and importing assets and a system for visual scripting called "Blueprints". It is available as a complete program and as uncompiled source code to allow for easy modification of core features as well as adding custom programming.
The source code for Unreal Engine 4 is available to all paid subscribers on Epic Games' GitHub profile and programming using the engine requires that you download and compile the engine directly, replacing the UnrealScript-coding from previous engines with direct C++ implementation.
Starting on March 2nd, 2015, Unreal Engine 4 is available to the public for free. along with a 5% cut of the gross revenue of every game sold. Publishing on consoles requires a custom licensing agreement with Epic Games due to the legal requirements of developing for the consoles.