*coughs and steps into the Demon Lord thing* Okay. Wind, Mika. As you said, Hell is 9,000 years old, who's to say that four--I'm sorry, no, it's [i]five[/i] [b][i]powerful[/i][/b] demons, why couldn't they dominate the regions of Hell? Moreover, who's to say that there weren't more kingdoms, but they got taken over by the cardinal directions. Maybe there are demon lord [i]level[/i] demons who want to overthrow the kingdoms. Also, we have [i]no[/i] idea [b][i]how big[/i][/b] Hell is. We don't have a map, we don't have scaling, and it has never been discussed. 

Maybe there's another continent to Hell that's like twice the size. [i]We. Don't. [b]Know.[/b][/i]

So, while I have no issue with [i]more[/i] demon lords, I do have an issue with Lords that were not previously [b]established[/b] and trying to take places from one of the new people that wants a lord. 

Oh, and let's not forget that adding a surplus of Lords to the already established kingdoms [b]would break continuity.[/b]

Also, it's also probably a very rare practice to share a kingdom considering that most demons are selfish. Why would they share something with someone who could possibly become more powerful than they? *points to previous rebellion point and other continent* Explore that avenue.

[b]EDIT:[/b] When I say level, I mean the power of the demon, not necessarily their status in Hell.