Best approach huge VR landscape?

So I have created a VR landscape which is about 4km x 4km.
It is running pretty smooth, but I think it can be more optimized.

I am still a noob and didn’t find any answer to my question yet, which is:

What is the best way to create a VR landscape that is optimized?

And by this, I mean: do I create a landscape that is 4x4, do I create different levels or different smaller landscapes and combine them? I have no clue at all!