I'm sure they will take a chunk of 4th and 5th books this season, and I think they've got to finish both of these books in season 5, maybe including something from the 6th book as well. As it happens, both 4th and 5th books are mainly world-building with not so many "oh-my-god" scale events. They are building up to at least two major events, yet due to the length and editing choices both events were left out of the fifth book, and I'm sure this won't work on the show.
GRRM himself said A Feast For Crows and A Dance With Dragons should take two or even three seasons, but I honestly think this is unrealistic as hell. Only a couple of faithful watchers will keep up with this show if this would be the case, so they obviously have to shorten all the stuff, especially as they currently have seven seasons plan.