The entire narrative drive of the books is the intrigue between the Lanisters, the Starks and house Baratheon. The moment they killed off Robb in book 3, that entire interplay fell apart. Everything from that point on was pre-ordained due to the logics of the power structure. House Lannister had to fall because house Stark did fall. Thus Martin wrote himself into a hole.
His argument for book 4, that he wrote so much stuff that he had to split things up, is poppycock. The fact is, he's written himself into a hole where the entire narrative interplay and suspense of the series is now missing. The reason book four is considered medocre is not because it contained characters that no one wanted to hear about. It was because nothing happens! Why does nothing happen? Because the entire conflict that drove the first three books and made them so interesting no longer exists.
The question in book five is, how can Martin bring that interplay back into the series. I think he's been trying to figure that out. Remember, the books were originally planned for three, and then he expanded it to seven. And while he had a mental construct of how to get from three to seven, I don't think he really thought it out. Thus, when he got the end of book 3 he ended up writing himself into a narrative hole.