It would suck, yes, but finding another coder would be way easier than finding an artist with the same style. Although the coder is responsible for a lot of the writing here too.
There will be a 5th chapter but it's an epilogue more than anything else. Because Mity only reads comments on his patreon, the only way to get more ATLA content is to join his patreon and be vocal about it. Right now most supporters ask about other, new projects and only a small minority (including me) asks about a refined book 1 and more content.
I doubt there will be a 5th girl though, you'd have to make up all of her story.
PS
You know Desna is a boy, do you?