So "Acarthasis: Contesting Tribes"?
I like that name as well
.
For theme, maybe it can work like this...
The pretender has pulled the four tribes together. Even though the tribes are bound together by the will of the pretender, they still jockey for position politically and have a distaste for one another, bred by their worship of different elements. The exception being the priest(s)(I am going to create) who are ordained by the new order, so they are not subject to the politics of the precarious alliance.
So while there is a truce of sorts mandated by the pretender, the factions still play the game of political intrigue, trying to stab one another in the back. To top it off, they do not work well when mixed together in units, and would rather be in legions of their own ilk (cold auras, hot auras, vulnerability to opposite elements etc)