There is this page from UDK/UE3 that is about BSP/CSG editing. I cannot recalling seeing anything similar to this with UE4’s documentation though.
https://udn.epicgames.com/Three/GettingStartedWithGeometryMode.html
There are also some modes that are no longer relevant here, but what is listed looks to still be edited the same way.
Maybe this will help in the mean time.