Just looking at the closest thing I can think of, in the NFL from 1990-2012 there has been 1 five seed and 2 six seeds that have won the super bowl. That means a 13% chance by those numbers. But the college game is different from the NFL. Realistically, the chances are probably not much higher but right now, without knowing how the end of the season shakes out, I see no reason why Clemson, Utah/Oregon winner, and OU (if they win out) don't deserve a shot at the playoff for potentially winning their conference.
Here's why I don't think we don't need to expand this thing-One, Alabama and Clemson (if they're engaged-I don't think they were when they played UNC) are on a whole different level from the rest of the sport. They don't have recruit-they basically pick the kids they want. Georgia's the only program in the country that you can argue is close to the two of them. You get 3-4 others who get to that level and then it makes sense to.
Two, I don't think we've had the issue with this system yet that we've had with the BCS where you have a team who got left out but had a compelling argument to get in over one of the four who did make it in:
2014 TCU/Baylor-Baylor won the H2H, so you can't take TCU over Baylor IMO. Ohio State played a P5 opponent in the OOC. Baylor didn't. tOSU also beat a Wisconsin team that finished in the top 15 that year by 59 points for the Big 10 title.
2015-didn't really produce anything worth mentioning here. Michigan State won at Ohio State and the Pac 12 produced a 2 loss champion.
2016-Again, nothing worth mentioning. Some debate over Washington's cupcake feast in the OOC, but no viable alternative. Oklahoma had two losses.
2017-2 loss Ohio State beats undefeated Wisconsin, which opens the door for 11-1 Alabama to go in as the 4th team.
2018-Oklahoma goes over Ohio State because OU's loss came on a last second FG to Texas (who they would beat for the Big 12 title) while Ohio State got nuked by a .500 Purdue team.
Here's the scenario I want if I want this thing to expand-Let's assume the SEC and Big 10 champs both get in. Let's also assume Clemson wins out (they'll get a shot to boost their resume when they get Wake at home and Virginia in their CCG). Then let's say Utah wins the Pac 12 and OU doesn't lose again. There's where you can really have some fun.