I personally would create the meshes in your 3d program and then put everything together in the UE4 -> e.g create a cliff/rock set and build the real cliff in the UE4 (you can use those assets again at different places) 
rough overview:
Materials/level creating process/light -> in the UE4
meshes/textures/collisions (depends on the complexity of your mesh)/lightmaps -> in blender