Top 2 from each conference, round robin playoff.
A 4 team playoff is ridiculous, and won't make more $ than the bowl system.
I just don't think anything more than a 4 team playoff is economically feasible, especially for the fanbase of the schools who would end up having to pay/travel 3 or 4 extra times (kind of a punishment for winning, in a sense), therefore turning the playoff itself into a bust.
If it was, we'd have one already (IMO).
As mentioned by someone else, keep the bowl system and add the 4 team playoff.