Don't know about the whole "lesbian thing". I can't say that I see it being completely out of character for Tara though. Seems like something that she COULD do. She might be scared away from sex/intimacy with men after her ordeal with that last vampire (Franklin? don't remember).
As far as following the books, it's too early to say that they're not following them (loosely at least).
The whole fourth book was centered around witches and the werewolf groups in the nearby town, if I remember right (it's been months since I read them).