As far as book sequence, Martin has said that he chose the sequence of 4 and 5 by grouping character stories by location mostly. The afterword in one of the books says that he basically wrote like a 1500 page book 4, then realized it was too rough ridin' big and that he had to split it. He couldn't find a good point in the overall time line to do so where it wouldn't be weird, so he split it by grouping those in Westeros and those outside Westeros for the most part, iirc.
So, once the next season is over, they could probably tackle 4 and 5 via timeline rather than geography. Doing so would make more sense for a TV show.