OK, I went in my project and messed with a few settings, and it seems skylights are WAY more important to UE4 than UE3. In UE4, skylights fill in reflection data that would otherwise be missed by the screen space reflections: the sky and ambient lighting all around. It can take any cubemap, or function as a basic reflection capture. I recommend keeping it on as a stationary light, as static lighting fails to fill in missing reflection data. For some reason, even sphere reflection captures don’t pick up the sky, so it’s really important to have one in on stationary mode.