4 bye games cuts the field to 8, not 4. The second round is another round of 8 teams, so 4 games. With 4 teams left, you have 2 games. With 2 left, you have 1 game.
The season, as it exists, is now 12 games. If you go through the playoffs, that's 4 additional games. That's a 16 game season. Simple math, not a point that's arguable. At the present time, with Bowl games, a lot of the teams already play 13.
The time will come that they will add another regular season game, count on it. Anything that generates more money. It's what it is.
And by the way, it's not 4 extra games for playoff contenders, it's 8. Math is math.