They do have the general blueprint from the creator of the whole thing, and the TV series has already overtaken the books. A book series that has only two installments left by the way, which means that the last two seasons will roughly cover the last book.
It started out one 10-episode season per book, so I think it's more than enough. Especially since good storytelling doesn't drag on and on and on and on just because hardcore fans of that world want more.
Actually, I think it's a good thing, since once the TV series went into uncharted territory so to speak, it became less character driven and there were these weird skips in time, making it seem like people more or less teleported. I don't need another four seasons of that.
read fluently, understand well, speak badly;
read fluently, understand badly, can't speak;
read some, understand a bit, speak a few sentences;
heritage language, want to understand and speak but can't.