But it's also possible that teams that would have been selected in the 5-8 spots would have won. Is it likely? No, but it's possible.
Also, with a 4-team playoff, it's possible for every P5 conference champion to go undefeated and someone gets left out. That wouldn't happen with an 8-team playoff. Plus, as it stands a G-5 team will never get in even if they are deserving. If you go to an 8-team playoff with auto-bids for the P-5 champions, 2 at larges, and the top ranked G-5 team, then everyone has a path to the playoff. You would still have complaints, but really the only team that could really complain would be the second highest ranked G-5 team. Otherwise it's " you should have won your conference or been one of the two at-larges."
It could also lead to better nonconference schedules because a loss wouldn't hurt your playoff chances nearly as badly.