Picking up on shvrdavid's last comment - The benefit of V4 4K face map is realised because people want to render portraits,  the area of the face fills most of the picture frame and the extra detail is used even when displayed on a computer screen. Compare that with a horse, most people will render that at a much further distance from the camera so making use of the extra LOD on a seperate head UV is more difficult to realise so of less use.

Care must be taken with multiple UV maps when shared via the Internet as they increase download size and what tends to happen is people compress the textures to support slow Internet connections and this has the overall effect of a lower LOD for the whole model.