4 teams is a disingenuous concept when you can leave out Conference winners from the “Big 5” (That in and of itself is stupid at 4 teams) in favor of 2 SEC teams (Did not happen this year for a change).
8 is the sweet spot; all Big 5 (Do the math, duh) conference winners will be represented, highly ranked teams that did not win, Notre Dame (If they are undefeated for the year), and teams like UCF will get in.
There is no sweet spot, all expanding to 8 would mean is transfering the complaining from teams #5 to #7 to teams #9 to #12.
In fact, due to how normal distribution works, there is probably a smaller gap between #8 and #12 than there is between #4 and #7, which means there will be more complaining than before.