my friend and i tried it the other day. we set it up as a 4 person game (starks, greyjoys, lannisters, baratheons) and each of us chose two houses out of a hat.
it ended up perfectly with the starks and the lannisters vs. the greyjoys and the baratheons.
the only way i would modify it is to block off the south as if it was a 3 player game in order to make baratheon come fight everyone instead of winning the game down south without ever engaging anyone (this is a huge problem with a 4p game too.)
it's basically as if the two houses on each team are just steadfast allies that never go littlefinger on each other.
it takes a long time and gets difficult when you're planning strategy in the later rounds with huge armies and two huge factions to look over, but it's still fun.