I think that there will still be a need for a 5th "Super Conference)".
No conference really can lock up an entire region with all of its major teams. The B1G is going to likely not get Notre Dame, the SEC can't absorb all of the ACC schools in their region, and the Big XII is doing just enough to stay relevant.
I believe the following will happen (lets just call this a random prediction):
1. The PAC-12 doesn't expand to 16. At best, they expand to 14 with teams like the aforementioned Boise and SDSU. No other major market teams exist in their footprint except maybe Nevada or UNLV - even then, their academics don't fit the footprint. BYU is also an interesting issue, as they don't really fit in anywhere with their magic underwear.. They'd seemingly fit in with the Pac-12, but I doubt the universities would want them, so the BigXII is another opportunity.
2. The B1G is the first to make it to 16 by acquiring UVA, GTech and potentially others while raiding the ACC (UNC). This causes the collapse of the ACC as we know it. If a 17th member is added (UNC), then the B1G eventually makes it to 18 by grabbing Pitt from the Big East (its the largest non-private school in the BEast/ACC). Other considerations would be Boston College and Syracuse to shore up the remnants of quality northern institutes.
3. With the ACC collapsing, the BigXII and SEC raid the remnants of the ACC/BEast for viable members. The SEC makes it to 16 by adding VaTech and NC State.
BigXII adds South Florida, FSU, Clemson, Miami, Cinci and Louisville to make a very viable "Eastern League" consisting of these schools as well as their recent WVU pickup for the east. This evolves the BigXII into a center and center-Southeast conference.
4. With the collapsed BigEast and ACC, they have no choice but to merge into the also-ran conference (ARC). The conference picks up Notre Dame as the cornerstone member, as ND seemingly only wants to play with someone they can control, much like Texas. Also helping out the new also-rans is the fact the ACC has traditionally raided the BEast for members.
This additional 14-team (21 w/ Basketball) conference consists of:
- Notre Dame
- Duke
- Boston College
- Wake Forest
- Syracuse
- UConn
- Temple
- UCF
- Houston
- Memphis
- SMU
- Tulane
- Navy
- East Carolina
- DePaul
- Georgetown
- Marquette
- Providence
- St. John's
- Seton Hall
- Villanova
The other conferences fight over the occasional playoff scraps as the MWC, Sun Belt, CUSA, and so on work together to try to get one team in an 8 or 16 playoff every few years.
Here's the map:
http://imgur.com/Tn8zr
*edit*
The one thing I would mention is that there is only one way we end up with 4 super-conferences: If the Pac-12 gets the TX/OK bloc of the BigXII (Texas, OK, OKST and either TTech or Baylor). In that scenario, the dissolved remnants of the BigXII would go to the ACC/BEast remnant conference, which would likely swell to ~20 members.
So you'd have the B1G at 18-20 schools, the Pac-16, the SEC-16, and then the Also Ran-20.