Blender -> you could generate a landscape in blender (like with world machine) which you can export as a heightmap. So it’s easy, fast and you get a pretty good result ?v=EnrrsVzkqnc (you have to use a raw 16 file for the heightmap)
UE4 -> you need a little bit of practice to get a good result, but the advantage is, that you can do everything in the engine: https://docs.unrealengine/latest/INT/Engine/Landscape/index.html