I'm happy with 4 but if we're going to change anything, I think 6 is the magic number. You can decide which 6 (5 conf champs + 1 at large OR just go 6 at large). There's pros and cons to each.
I get people want more games but it's diluting after a while. Even an 8 team playoff, watching the 1 & 2 seeds massacre the 7 & 8 seeds isn't entertaining. 9-3 non-conf champion teams shouldn't be competing for national titles anyway.
Also, 12 teams means it's going to end up being 4-5 SEC teams in it every year which is just boring.