If there would be a 4th tier, it would mean that an empire would always consist of a number of vassal kings. I don't think that was historically the case. It would make more sense to have the emperor title as a boosted 3rd tier title, with some advantages over kingship such as increased prestige.
Also, if barons constitue a tier in the hierarchy of CK2, kings will be already 4th tier
+1
There are basically two issues:
1) Should there be a fourth tier?
2) What should being an emperor entail (ie should it just be a trait or should it give access to a fourth tier)?
I've already voiced my opinion on the second of these issues.
Assuming the basic mechanics of vassalage and the size of the map remain similar in CK2, I increasingly dislike the idea of a fourth tier on a mechanical basis as it gives the holders of titles on that tier MUCH more potential power. The amount of time spent dealing with rebellion in CK1 is directly proportional to the number of vassals you have. For the sake of argument and simplicity say a duke can cope with having 10 counts as vassals before rebellion becomes endemic. Likewise, say a king can cope with having 10 dukes as vassals. The king potentially wields power that is an order of magnitude greater than that of the duke. Introducing an "Emperor" (or whatever) tier gives a figure that is potentially 10 times as powerful as a king. At the same time, personal demense would be doubled for an Emperor compared to a king giving an emperor another huge advantage. Both of these would make it much easier to control a massive territory and face little opposition from those outside your empire. A massive internal revolt could still cause problems, but then you should be more powerful than your vassals, and have loyal vassals to call on.
In CK1 you can build massive and relatively stable empires by granting multiple duke titles to your vassals. However, the power of these archdukes, and therefore the power you could draw from them as king, was limited by their demense and how many vassals they could cope with. You could also get occasional problems when an archduke was inherited by a foriegn king and you lost a massive block of land.
I therefore dislike the idea of a fourth tier as it would allow an already powerful king to become a figure that couldn't be touched by anyone. If the map were to become much more detailed (say a tripling of provinces) then I could see the benefit of a fourth tier. Likewise, there may be a use for one if the system of vassalage is overhauled. At present I can only see one viable use for it: the Mongols (as a few people have already suggested).
By giving the Great Khan the fourth tier then he has the potential to grab and hold a lot of land. BUT the amount of land that he can personally control would remain finite. This is in stark contrast to the unending amount of land he can hold under the present "if you're a mongol, then you can personally own the whole map". After a period of time or the deaths of a certain number of Khans then this privilege could be removed leaving a number of smaller Khanates.
