This patreon post talks more in depth about what's planned with the girls: https://www.patreon.com/posts/how-game-will-37415628
In short, Celine and Leah will be the main girls and the others (Maddy and Violet) are side characters. There are 3 endings depending on your choices (the linked post explains these endings).
Glad your enjoyed my game, you can join the discord server / become a patron if you'd like the support me! :)