@Shdill explaining all of that is much longer then i want to get into since im a lazy f but generally western style nations will be king - prince - duke and eastern style nations will be emperor - prince - duke. i dont see the emperor - king - prince - duke set up that often and normally its only used to flex on the other nations how large that empire is. after i thought about it a bit the holy roman empire and byzantine empire probably used the emperor - king - prince and/or duke set up.
historically, i believe it was done due to how hard communication was after expanding beyond a certain point so it was easier to just grant them autonomy while also promising to help each other once bad things happened.
but if you dont wanna deal with that minor issue and your world has magic / cultivation you can solve it easily so your nations can be much larger then their real world counterparts. talismans, orbs, soul lanterns, some sort of mirror thing, and even texting have been used to chat across large areas.