Books 4 and 5 run contemporaneously... and while book 4 is thick, book 5 is thicker. If they're splitting books, those may 5-6 seasons total.

Then again, i think there's a bunch of stuff that could probably be cut for a TV series. Like Tom Bombadil in LotR, there's some stuff in there that doesn't really add or take away from the story.

Related to Book 5 (spoiler):
Like, do we really need the entire Reek/Theon and Ramsay Bolton story line?