Of course it depends very much on what faction you are playing as well. For example southern Italy cities work best using 3's and 4's at first because you can get the better troops from them even though they are Roman Homeland provinces. Furthermore, Ireland for the Casse is Homeland (or expansion maybe) but benefits from 3's and 4's due to the goidolic (sp?) troops.

Generally type 3 governments suit me fine for whatever i need especially cosinering that I don't care much where I move my generals and always forget and send them into type 4's by mistake (a big no-no!).

Use the recruitment viewer and see what troops are exclusively available to lvl 5 reginal MIC and thats the provinces that you must have a lvl 4 in to get that. Otherwise its down to personal playstyle.

NB: I know most factions can get Elite Thracian Infantry with a lvl5MIC. But as far as I can remember the ONLY other units that the KH get with that level MIC is the Noble Gallic cavalry over most of Gaul and the Scordisci troops in that wierd gallic province on the western edge of Thacia. Other than that you can build KH lvl 3's everywhere else and not miss anything.