Definitely four or six...I guess it depends on how many weeks you want to use for playoffs and which weeks you want to use.
I think you are right LooseCannon...I like doing the playoffs with just four. It actually gives the teams who did well during the entire year a better chance to win. If you are in the top four of a 12 team league, you probably had to win 10 games to make the playoffs. Your best teams will be in that group no matter how much luck they had in winning a few games here and there.
Sure, the NFL has 6 in each conference, but they also have 15 teams. If it were a 14 team league, 6 is good...but I like four for a 12 teamer.