As I suspected that it was low rez on the lightmaps. Alt+0 is good way to troubleshoot bad lightmaps, sometimes you can see that the uv is not that “flat” that you think it is. For the UV layout, the less space the uv islands cover the bigger resolution you will need to bake nice shadows. Also the bigger is the resolution, the longer UE4 will bake the light. I think its a bad practice that a lot peoples do to have all the walls inside one uv space. The best way (tho its a little bit time consuming) is to detach all the walls faces and apply uvs on them, this way you will get very dense lightmaps on lower resolution. 