Doelger was interviewed backstage at the BAFTA Awards and said the following:
"[The number of series] is being discussed as we speak. The third season was the first half of book three, season four will be the second part of book three. George RR Martin has written books four and five; six and seven are pending.The potential length of the series has been much-speculated about by fans, with the need to adapt the Song of Ice and Fire novels (relatively) faithfully clashing with the realities of TV production and how long a TV audience will wait for answers and resolutions. Previously showrunners David Benioff and D.B. Weiss had spoken of 90 episodes (nine seasons), whilst Doelger's comments seem to suggest that less is more likely. Indeed, with Book 3 spanning two seasons and seven books projected, it's telling that anything less than eight seasons (assuming the remaining books can all be fitted into one season each, which is not a given) is even being considered.
"I would hope that, if we all survive, and if the audience stays with us we’ll probably get through to seven seasons."
At the same time, even highly successful shows like Lost have ended after just six seasons, and shows like Star Trek: The Next Generation and Buffy the Vampire Slayer only managed to limp to seven years with a sub-par season (or two) thrown into the mix. True Blood arguably ran out of steam after three (four if we're generous) seasons and has been running on fumes ever since. So the chances of Game of Thrones - a massively expensive and time-consuming show to make which doesn't allow its producers much (or any) time off between seasons - making it to eight or nine years without burning out seem slim, so aiming lower and writing accordingly might be the better option.
To me, this suggests that the producers are envisaging a more radical move away from the books after Season 4 (if not this season), with perhaps more focus on bringing the existing characters and storylines to a resolution rather than bringing in lots of new characters and situations. Indeed, Benioff and Weiss previously said that Season 3 will be the peak of the cast in terms of size and future seasons would feature smaller casts. That suggests that we might not be getting the hordes of new characters the later books bring in, and casts doubt on whether we'll be going to locations like Sunspear where a dozen or more new characters would need to be introduced in one hit. This may improve pacing and clarity, but could backfire if some of the other storylines (particularly in Dorne) prove critical to the overall resolution of the story and the producers have to change their minds later on.
That the show would become much less faithful of an adaptation when it hit the problematic (from a structural point of view) fourth and fifth books has largely been expected by fans, but to get to the very end of the entire storyline in just forty more episodes after this season will require more compression and rewriting of the novels' storyline than what we have seen so far. If HBO and the producers can pull it off remains to be seen.