If the seasons follow the books I don't think there will be a fourth. I was told they are filming the third now or I would guess it wouldn't make it past two. Hopefully I am wrong.
What do you mean it won't be a fourth? They are definitely following the books, GRRM has said on several occasions that's important to him and that's how it will be done. The way the third book ends, there is no way they can do a line there and say that's it, the fans would riot. The show has a strong cult following and runs on a station that cares about making good TV, and doesn't like cutting shows prematurely. HBO stopped producing Rome after season 2, which they later admitted it was a big error.
As long as the wieving figures stay on a acceptable level GoT will continue to run. More interesting is what they will do with the third series, which is based on a massive book, with A LOT of shit going on. Hopefully they will do a 16 episode season or something, that would be great.