The big problem is how to handle the geographical division. This works very well with 5 leagues, because the regional traditions and special features are included without the travel costs and travel distances getting completely out of hand in a league. At the same time, the different club density and population density in Germany is taken into account.
With 4 leagues there would be more conflicts. I can’t think of a fair solution either. If you divided it into north, east, west, south, it would be unbalanced. In northern and eastern Germany there are around 30 million inhabitants, in western and southern Germany 54 million inhabitants. That has to be reflected somehow in the distribution of the leagues. At the same time, nobody in Baden-Württemberg, for example, wants the federal state to be divided between two leagues, the same in East Germany.
Theoretically, you could, for example, combine Bavaria, Saxony and Thuringia, sort of like the League of Free States, which would represent around 20 million inhabitants, which is great. But then teams like Chemie Leipzig and Wacker Burghausen would have to drive halfway across the country for the away game.
The Ostclub-Regionalliga plus Berlin has to stay one way or the other because they can make the sometimes very large distances palatable with the East identity. RL West, for example, is very privileged in this respect and does not have to cover such distances. And such artificially slapped together leagues like the “Freistaat league” have zero identity and marginalize organically grown fan culture in the form of fan friendships as well as fan rivalries.
Conclusion: Personally, I would actually be in favor of increasing the 3rd division to 22 teams with 5 relegated teams