^Agreed. There's also the possibility of the TV series taking a year off between seasons, as HBO shows sometimes do. I think Benioff and Weiss have said they anticipate an eight or nine season run, which suggests they expect to spend more than one season on at most one of the four post-Storm
books. They may also be thinking of combining Feast
into a single season (perhaps with a little spillover into the seasons preceding and following)-- even given their combined page count, it's hard to imagine getting two years' worth of satisfying story out of those books, let alone more.