It is actually completely possible to create a Light Vector in the UE4 editor, I’ve done it and it’s working.
The difficulty is if you want multiple lights, this would be possible as well, but a little difficult. It involves using the insanely powerful new Material Parameter Collections, and using a blueprint actor in the level to continuously (well, if the light is ‘moveable’ or ‘stationary’, otherwise you only need to fill the values at begin play) update the vector parameter with the vector of the directional light, or whichever dominant light you choose. On a per-actor basis, you could even calculate the light vector for multiple lights in the blueprint before outputting it to a parameter in the shader.
Post process is probably slightly cheaper computationally, but it’s good to know that the old methods CAN be made to work!
To see a working example of this, open the blueprint, and the material for the BP_SkySphere in engine assets, or start a default level and grab it from there.