Epic Games has released the Photorealistic Character sample: a free textured model of a male human head designed to demonstrate photorealistic skin, hair and eye shading techniques in Unreal Engine 4.
The model, which was captured using Otoy’s facial scanning technology, comes with 4K skin textures.
Epic Games has also posted a thorough explanation of how the shading was set up in its online documentation, which is worth a look even if you aren’t using UE4.
The Photorealistic Character sample is a free download accessible via the Learn tab of the Epic Games launcher. The Launcher is available for Windows 7 and above and can be downloaded here.