Penn State or Ohio State seem to have the best shot in the Big 10....but Minnesota is still undefeated, and if they manage to win out they'll make things interesting. Ideally Ohio State & Minnesota win out and the Big 10 ends up with 2 undefeated teams playing for conference title, then they can make a case to get 2 teams in.
Clemson is your closest "lock" to stay undefeated. Wake Forest is their stiffest remaining competition, and it's a home game.
OU losing to Kansas State has put a wrench in their making it I think. Baylor is also undefeated...so who knows how good they are they get OU at home though. SEC should end up with at least 1 team as a 1 loss once the dust settles, potential winner of LSU/BAMA can remain undefeated too. If Florida beats Georgia they are basically locked in for the East and a 1 loss...Georgia wins they still got A&M and Auburn to play.
Oregon still has an outside chance for getting the PAC-12 in...they win out and win their championship game their only loss will be first game of the season to Auburn. Most likely they sneak in.
If I was to put any money on the table though...the only 2 teams I'd bet on being in the final 4 right now? Clemson & Ohio State