I don’t know what Unity’s magical slider does. There is no such slider in UE4 because it goes against the main idea of PBR - Physically-Based Rendering, where the shade of your objects in or out of shadows is based on the interaction of surfaces with the light. If you ever did photography that knowledge is super useful here. There are couple of articles you might want to read to better understand what is going on:
http://blog.selfshadow.com/publications/s2015-shading-course/
One thing you seam be missing is auto-exposure or tone mapping Auto Exposure (Eye Adaptation) | Unreal Engine Documentation
it goes hand in hand with PBR especially in bright exterior scenes.