You can change the Z value. If i remember correctly, you need to select the landscape actor that is being created, not the the proxy's them self. If the values are greyed out you need to right-click on the landscape actor and ''unlock transform''. now you can change the height.
All exported tiled terrains import with large negative Z values. Tried setting the base height and maximum height to many different values (0/512, -512/512, 0/2048, etc). All tiled exports to R16 result in the base terrain around -26000 Z.
What's worse UE4 does not allow for height adjustment in world composition.
Exporting to a single landscape and then adjusting the height works fine. However I am working on a large tiled landscape so that is not an option.
Anyone have hints for settings to use to get the world to appear at Z 0? Procedural landscape stuff doesnt work with large negative Z, for example, and lighting is screwed.