Letter four completes the first cycle of the four elements. Water symbolizes the subconscious side of the mind where we absorb and assimilate the results of our experiences with the other three elements. Habits live here, and most people operate on automatic pilot most of the time. The subconscious manages the body. It digests our food, repairs damage to the body, keeps us breathing while we sleep, stores information acquired in the past, etc.
Habits are great labor savors.
Once we learn to do something, it takes less attention and effort to do it again, so we can satisfy the fire urge to break new ground. I'll try it and find out. Earth is basically practical. Do it and see. Air is basically logical, creating abstract conceptual systems, maps and models of ideas.