The folks at Unreal recently held their Unreal Dev Day in Montreal, where industry experts touch points on best practices and techniques within Unreal Engine 4. One particular topic they gave insights on was Lighting within Unreal Engine.
Jerome Platteaux from Epic games gave a tutorial, or a class rather on using lighting within the engine. He touched base on lightmass, which is used to create lightmaps with complex light interactions like area shadowing and diffuse interreflection withing UE4. This will hopefully help upcoming developers to grasp the concept and produce better light bakes within Unreal Engine.
I would highly recommend watching the full video.