Exactly how does a 4-8 team playoff make the regular season less meaningful? You'd still couldn't lose more then 2-3 games. It would be pretty much the same as it is now, except the other 2 loss teams would get a chance to play.
One game has already made the regular season less meaningful. Since the BCS came along conference championships have taken a back seat to the BCS Championship game.
Even conference championship games have hurt the regular season. I mean you have had 2 or 3 loss teams playing in a championship against an undefeated and 1-loss teams. That diminishes the regular season.
A playoff will further diminish it.