Hi. Simple question. I have 4 deck of cards and 6 houses. So can we play with 6 players ? How should we choose deck of cards ?

Add two more decks and you can play with six players. With the four you have, most you can do is a four player game. If you don't have the Kings of the Sea and Princes of the Sun expansions, I recommend them. That will give you your other two decks for houses Greyjoy and Martell.