ranked conference champions getting the byes. Those top 4 spots aren't reserved for the power 4 Champs, they are for the 4 highest ranked conf Champs.
ie, let's say the eventual conf Champs for these 6 conferences were ranked ad such:
B1G 13-0 Oregon #1
SEC 11-2 UGA #5
Mnt West 12-1 Boise #9
ACC 10-3 Clemson #13
AAC 12-1 Army #16
Big 12 10-3 Colorado #22
If the above scenario played out, B1G, SEC, Mnt Wet, and ACC would get the byes, the AAC would get auto bid at 12th seed as 5th highest Conf Champ and Big 12 would watch from home eating popcorn.