Fun fact: there originally were five

Was the fifth inserted into the wheel somewhere, or was it some sort of element "in the middle," so to speak? Or is that something to reveal later?

It was along the wheel! The old elements were air -> fire -> ice -> nature -> water -> ... didn't make much sense with the extra phase of water in there though, and interactions between the tribes got a little complex, so i merged ice and water into the same element, given they're just phases of the same thing. Plus four tribes mirrors the four races! Makes it more consistent to have numbers repeating - "four groups" is something that will keep showing up.

Ah, I see. That explains the fire -> water, then; I was wondering, because it felt atypical.

From the perspective of the Lagre, quenching of a fire with water is really just starving it of air, after all - there are chemical fires that can't be stopped with water; starve a fire of air, and it will always stop burning. Similarly, heat (or absence of) changes water between forms. They used fire to boil water, to melt ice, and so on.
I figure "fire trumping water" is kind of an uncommon stance to take in an elemental system, but the elemental cycle is a very good reason for Lagres to persue science - after all, they're trying to understand why the cycle is how it is. Nature trumping Air would be something they wouldn't understand fully until the discovery of photosynthesis, for example.