I'm real big into College Football Reference's SRS which uses strength of schedule and margin of victory to determine their rankings.
Right now, the top 4 in their rating are Bama, Clemson, Ohio State, and Michigan. The rest of the top 10 are Notre Dame, Oklahoma, LSU, Stanford, Baylor, and Florida. Michigan State is ranked 15th, Oklahoma State is ranked 17th, and Iowa is 20th. USC is actually ranked 11th due to their strength of schedule (5th) and apparent margin of victory in their wins. Obviously Michigan has two losses and shouldn't be considered for the playoffs even though the Wolverines lost two close games at Utan and Michigan State. The top 5 offenses are Oregon, Texas Tech, Baylor, Memphis, and Notre Dame while the top 5 defenses are Michigan, Ohio State, Wisconsin, Alabama, and Florida.
http://www.sports-reference.com/cfb/...5-ratings.html (You can click on OSRS for offensive rankings and DSRS for defensive rankings)
Strength of schedule helps Bama (2nd) and Notre Dame (4th) and hurts Iowa (74th), Okie St (80th), and Baylor (109th).
http://www.sports-reference.com/cfb/...standings.html (Click on SOS for strength of schedule rankings)
Conference rankings have the SEC as number 1 followed by the Big 10, Pac-12, ACC, Big 12, and AAC. While there is a big gap between the power 5 and the AAC, these 6 conferences are rated as above average while the rest of the conferences are below average with C-USA being ranked the worse this season.
http://www.sports-reference.com/cfb/years/2015.html
My top 4 right now would be the 4 teams ranked with Clemson, Bama, Ohio State, and Notre Dame. I think an undefeated Big 12 team will most likely get in, but if Notre Dame does win out and wins at Stanford to end the season it would give them quality wins against GT, Temple, Pitt, USC, and Stanford with their lone loss at Clemson by only 2 points. Bama needs Florida to keep winning as well to give them a quality opponent in the SEC le game. If Oklahoma hadn't loss to Texas, they would easily be in the top 4 right now.