They'll be combined, but not into one season.
Right, that's what I meant-- big chunks of Feast
in particular take place during the end of Storm
anyway and will probably appear in season 4, and they may end season 5 at the point in Dance
where the two narrative streams unite and leave the later developments in the Meereen, Braavos, and southern Westeros stories until season 6.
There's no firm plan for the length of the show's run; at different times different producers have mentioned 7, 8, and 9 as possibilities. Obviously a lot is going to depend on the point at which they have to go it alone based on the general information Martin has given them.
The Winds of Winter
certainly isn't going to come out before they start scripting season five. Six scripts for season four were already written by March of this year, so they're going to be scripting season five by the beginning of 2014. Based on the tone of Martin's responses to questions about the next book, he's not going to be done by then.