As far as i can see, the Fog of war approach from this video can be reasonably adapt to your goal. In short - rendertarget applied by postprocess over whole world. UE4 - Tutorial / HowTo - Line of Sight Vision System - YouTube
Of course it still depends on your world resolution and the acceptable ways of visualisation, but given no details about your graphics - i can’t say if this way is applicable to your visuals.