As far as I remember, season four will be the rest of Storm and part of Feast and Dance, and season five will probably cover most of the rest of Feast and Dance, meaning we'd be into Winds sometime in season six. But I could be wrong about that.

Regarding the unpublished portions of the story, Martin has told the show producers the basic outline of what's going to happen in those books. If the show passes the books, the show will continue and will build its own stories based on that outline. The stories would certainly be a little different, but they would be able to hit the broad strokes of the ending.

The real question is, how many book fans would continue watching the show if it passed the books? I'm not sure that I would. I like the books quite a bit more than I like the show, and get a lot more satisfaction from them. If possible, I'd like to experience the rest of the story through the books first, before I watch the slightly watered down adaptation. The show would still be there to watch later on, after all.

But I wouldn't be surprised if I were in the minority on that.