If I may have word of advice.
I wouldn’t focus on looks or materials at this point. You have set yourself to monumental task (4k terrain!). That terrain must somehow populated. With things. Lots of things.
I would start from researching how to populate this area with minimum effort. Creating procedural blueprints which will automatically spawn assets and how to create content to make procedural world possible.
I can tell from experience. If you are planing to populate it by hand… You will quickly burn out and throw it all to hell (;.
Edit:
For this big level, you don’t want to use lightmaps. Really. Even Daniel Wright in one thread advised against it ;p.
Even if you manage to build it. Lightmaps will eat obscene amount of memory, while loading.
My 4x smaller level, with lightmaps build, is about 500MB. And it’s not even 1/4 finished.