After reading the first chapter of all four novels, here is what I have gathered. Since I used Google Translate, there might be some inaccuracies

☀️Earth – Water – Air – Fire are four cousins from the Watinwanich family.
☀️Earth is the oldest, followed by Water, Air, and Fire.

🌎The main couple in THE EARTH: 
Kasama (a.k.a. Earth) & Tippapha(a.k.a. Rose)
[A farm owner & Childhood friends (Daughter of the farm owner next door)] <This series will include action and violence scenes.>

🌊The main couple in THE WATER: 
Apo (a.k.a. Water) & Cholada
[Heir to the Watin hotel chain & Daughter of the Wajra family (a competitor to Watin)]

🌪️The main couple in THE AIR: 
Wayo (a.k.a. Air) & Catherine
[Lieutenant & Princess of a small kingdom near France] <This series will include action and violence scenes.>

🔥The main couple in THE FIRE: 
Achima(a.k.a. Fire) & Kaprao Athitaya
[Heir to the Watin family in Southern Thailand & Daughter of a worker in Achima’s house]