The main element of an open world is actually its landscape. UE4 offers a wonderful system that enables you to create any kind of terrain for your world (mountains, valleys, sloped grounds, caves, and holes for lakes or rivers) and easily modify both its shape and its appearance by using a range of tools.
The landscape tools are accessible by the Modes panel by clicking on its icon (it has an image of a mountain):
Manage: Enables you to create new Landscapes and modify Landscapes components. This is also where you work with Landscape Gizmos (a tool that allows you to manipulate volumes of layer data of your landscape to easily replicate or export).
Sculpt: The main mode of the Landscape editor. It enables you to modify the shape of the Landscape by selecting and using specific tools.
Paint: Enables you to modify the appearance of parts of your Landscape by painting textures on it, based on the layers defined in the Landscape's material.