This is my basic position as well.I think 4 teams is the right amount honestly.
The 4-team format makes it so in situations where there isn't a clear 1 & 2, or there are 3 teams with strong cases to play for the title (e.g., if 3 P5 teams finish 13-0 like in 2019), they all get a chance to compete for the title.
There hasn't ever been 4 teams who were legitimate title contenders in the CFP era, in my opinion, so I don't want to see a playoff expansion. Only 3 of the 14 total CFP semi-final games that have been played were truly competitive (2014 OSU-Bama, 2017 UGA-OU, 2019 OSU-Clemson). Also, I don't think any true title contenders have been left out.
Expanding the playoff would kind of be rewarding teams who didn't earn it by giving them a shot to play for it all. It would also likely lead to more rematches of rivalry games that should really only be played once a year. Of course if two rivals are the country's best teams (Bama & LSU in 2011), I have no problem with a rematch, and the current format allows for that while still having two other spots open so people can't complain.
There are also plenty of programs with a good enough mix of name/brand, history, and resources to compete with Bama, OSU, and Clemson on a more consistent basis that have failed to do so -- some examples include Florida, Florida State, Texas, Oklahoma, USC, Michigan, Georgia, LSU, Notre Dame.
Having 4 means you don't screw over an unbeaten number 3.
The end.
2019 would have been a horrible train wreck but for the playoff. And probably another split national championship, too.