Just a thought... Most Nolix layouts aren't a constant grade, but have flat areas at yards, towns, etc. Anywhere you have switching going on. Otherwise you get rolling stock taking a loooong roll to the bottom anytime you decouple them. Also, realize that _starting_ on a grade from a standstill is a whole different world from entering a grade rolling and maintaining momentum up the hill (more on that in a moment). So while 4% grade around all the walls at a constant grade would give you the separation you want, it might not be operationally feasible.
Personally, a 4% grade - certainly a consistent one - wouldn't work out for me even if that was feasible. For my taste, even with Challengers and the like, you'll limit the length of your trains more than I would choose to. I'd strongly suggest you find a way to "test" your motive power and rolling stock on a 4% grade before committing to it. Find out if you can run trains of a length that will work for you -- you may find that limiting yourself to 8 reefers or 3 pullmans, for instance, is going to be a recipe for disappointment.... Maybe someone in your area has a layout with steep grades?
Or, one thing I've done in the past, is just set up a long, straight track on a 1x4x12' board. Then you can "prop up" one end of the 1x4 to various heights to give you any grade you want. Instant grade-compatibility test for your equipment.
It doesn't need to be all that long as the ability to start on a grade is the acid-test anyway: I found 12' to be fine, even for testing multi-80'-pullman trains. Basically, if you can fit your whole train on the track and even have just 2-3' extra to "run" it, you'll get a good idea how it will perform in the worst-case scenario - sitting on a grade and need to start from a dead standstill. You'll certainly want to test start-from-standstill for your equipment if you're going to do a constant-grade nolix, as anytime you do stop, you'll be restarting on that grade...
Overall, I would have to fall in with those who mentioned a two-decks, two-layouts arrangement where they are not physically connected.
I did see a quite nice solution to that situation. A friend had a two-deck, non-connected layout. The upper level was logging and mining, and terminated into hidden staging. On the lower level, he had one logging and one mining train identical to the two he ran "upstairs", held in that level's hidden staging. So the logging train, for instance, disappeared into the tunnel on the upper level and a few minutes later "reappeared" on the lower level. Just a thought on how two physically not-connected decks don't have to be "not connected".
Though it was amusing on all-too-frequent occasions when he would unload the logging train on the lower level, forget to unload the one in staging on the upper level, and have logs 'magically reappear' when the train 'returned' to the upper level.