That depends on how open and diverse each level is and how much graphics memory the target platform has.
4k textures look good. If you have four walls and a roof, a table and chairs, and a couple of characters, 4k (With lower resolutions as options of course) is a reasonable choice.
4k textures take up a lot of memory. If you’re texturing the rocks in Skyrim in 4k, it’s probably not worth it for right now.
In other words, I’d continue to offer 4k textures, but I’d have them default to 2k and maybe have 1k options available as well. That’s just my opinion though.