I'm really sorry about your situation. But it seems like in your heart and head you know that protecting an innocent 4 year old from this is the right thing to do. And it is. As a mom myself, I know that there is something in us parents that makes us do the right thing for our kids when if we were not parents, we'd choose differently. But thats a good thing, IMO!
That is great you are going to Al-Anon. And I agree with what the others said - remember that she may be an addict with a disease, but she can choose to get help and if she wont do it there is nothing you can do. (Step 1)