Hidden stage cant help here. Game cant sort teams in stage as you wish. Best way for level 4 (level 3 also if i undersand this) is to use regional boundaries for competitions but you have to check all cities. Level 5 is first level where you can use my settings but problem is this : if some team in level 4 is in wrong division you can have problem with mathematics for relegation. I think this can be done but i need to change "teams" section in your case.
As you can see i use "ignore teams" option to ignore teams from other regions in "relegation zone" stage. In your cae i need to exclude all regions and to add relegated teams from all level 4 groups because i dont know where some team can be located based on "team move" action.
Very complicated system.
Also im not sure how regional boundaries option works on FM17. My serbian system is same in every level with child divisions so i use dynamic relegation without any major issues. I have level 6 system without single team in wrong division even after 20 seasons. Try to add all your clubs to level 3 parent divsion. Set boundaries for all 3 Serie C competitions and untick "use regional divisions to organize teams" in parent division. (active option but untick).
Test how teams are sorted in season #1 and season #2.
This is very big job and first think are you motivated enough for something like this
For example i have working system but lower league finances are major problem for me. Serbian clubs simply get too much money and i cant set that.
EDIT: for promotions to level 5 you can use divisions without any fixtures with fixed promotion number. I used divisions (every level 5 divisions should have their level 6 lower league).
So just to clarify- its big job and you should make plan first (i can help with this because its most important part) and start to make database changes.
Also you should test that boundaries so you can be sure how game will handle sorting.
EDIT2: hmm one more optional problem. I dont know how many relegated teams from level 4 can be from certain region because game can move teams to fill the gap. Thats why "relegation zone" stage is very hard to make.
That "use regional divisions to organize teams" option (i like to call it "hard regional rules") is made to keep teams 100% in their regions. Can be used in leagues with variable number of teams (IMO bad option becase you can have 24 teams in north and 12 in south after couple seasons - this is just example) or can be used in my dynamic relegation system. Its perfect for my system but this system cant be used in every country.
Thats why i think you shoudnt use my system because you cant use it in every level and can cause problems.