This is a feature that I’m surprised to see not implemented.
Coming from Unity where I could whip up terrains at will during runtime, change heightmaps, manipulate materials and do just about anything from code I find it strange to not be able to do anything of the kind in UE4. This is pretty vital to large scale games where you need to generate terrains while the game is playing and creating them by hand is practically impossible, or you want the output to be randomized. Currently the only option would be to manually create the meshes for the ground, which isn’t ideal at all, we lose all the advantages that the built-in terrain would bring with it.
Please, please let us alter terrains from code!