I actually think 4 is the right number, but 8 is absolutely too many. The college regular season is where teams prove who is best. The playoff should be to determine who wins out of those who can make that claim - and there are rarely more than 3 or 4 who can honestly do that.
Consider this: We go to 8 teams, which effectively means that every major conference winner (and some runner-ups, ugh) will make the playoffs. Entering the last week of a given season, OSU is undefeated and has the division wrapped up since Michigan already has 2 losses. How deep into The Game does Meyer play your starting quarterback? What if your left tackle is nursing an ankle? I'd rest both of them along with anyone else who needs it because this game (THE Game) now means nothing. THAT is how the playoffs will ruin college football. Before you dismiss it, go back and watch the '09 and '10 Iron Bowls. Bama and Auburn both had their divisions wrapped up those years, and eventually went on to Nattys. Both also trailed significantly in those games before epic comebacks when, had they lost they would have been out of the running for the NC. What turned out to be two of the greatest games played in recent history would have been throwaway games with a large playoff.