Generally there are only a handfull regions that justify a level 5 regional MIC. The main advantage of level 4 governments remains the speed at which you get a stable region. judging from my RV(7-factional;21-regional) there is only ONE unit that requires a level 4 gov and thus a level 5 mic the Cordinau Orca, right to the west of your homelands^^

Governments restict the construction of MICs and other buildings
gov 1 = Factional MIC level 5 ; Regional MIC level 2 ; all other buildings
gov 2 = F-MIC 4 ; R-MIC 3 ; most other buildings
gov 3 = F-MIC 3 ; R-MIC 4 ; some other buildings
gov 4 = F-MIC 2 ; R-MIC 5 ; few other buildings

Some factions (I only know of 1, Carthage) have a slightly modified system but that's a basic guidline. The Steppe people have a "totally different" system.