Hello everyone…
Lately I started painting my landscape with a lot of materials but spefiic '‘squares’'of the map was gettng black and some others were glitching!So I though to delete my long-hour work by clicking ‘‘clear’’ at material details…So one again the problem got worse as you see in the pictures cause some of the basic material is still there and those grey squares are changing '‘roles’'each time i move camera…if i click the landscape everything is normal but if i click any mesh this happens…!
Can anyone help me or i can not continue with my project. 
You might have too many textures in your landscape, I think right now UE4 only supports 6 landscape textures total.
Somehow I had 2 layers one above other so i deleted the first one and solved the second problems…the problem with the black spots is already in the document of Unreal(the section with Materials) 
Somehow I had 2 layers one above other so i deleted the first one and solved the second problems…the problem with the black spots is already in the document of Unreal(the section with Materials)