The terrain itself does not need to be streamed (divided into seperate files) You need to learn about streaming the foliage and rocks to make a bg map. Check out my examples maps to get started: Duke's Free Heightmaps - ARK: Survival Evolved - Unreal Engine Forums
Also, here’s my post with a download to the actual ARk map ‘The Island’ so you can look at that to get n idea of how to set your specs in WM: My New Map! - ARK: Survival Evolved - Unreal Engine Forums