I think you're making the same mistake everyone did when they presumed 4 16-team conferences - focusing on an end state without any realistic path to get there.

TL:DR, rooting for 4 equal team conferences without the PAC always ends poorly for the Big 12. Instead the best hope should be for the ACC (rather than the PAC) to fall apart

You're imagining 4 20 team conferences. Which means 80 teams. Currently there's only 69 P5 teams (including the 4 new), so you're imagining they add another 11, which seems a lot and frankly unlikely - San Diego St, Boise St, SMU, USF, Colorado St, Louisiana, Tulane, UNLV... it's getting pretty ugly already and I'm only through 8. With the Big 12 clearly weaker than the SEC, B1G and ACC, most of these teams would end up in the Big 12.*

Most commonly you'll see people envision 4 16 team conferences. But that doesn't bode as poorly for the PAC12 or as well for the Big 12. With the Big 10 already at 14, it's very unlikely they take just 2 west coast teams. It would more likely be poaching ACC teams (or ND, with Big 12 an outside possibility). This would leave the ACC needing to backfill...which would almost certainly be from the Big 12 ... and leave the PAC12 strong enough to take from the rest of the Big 12.

I suppose 4 18-team conferences could happen and that has potentially the best outcome for the Big 12

Scenario 1:
If the Big 10 raids 4 PAC10 teams (USC, UCLA, Oregon, Washington) it would definitely leave the PAC12 weak and it would be no surprise then to see the Big12 grab Colorado, Utah, Arizona, Arizona St, Stanford and Cal to go to 18, with Wazzu and Oregon St left to twist in the wind.

But if the SEC and ACC also decide to go to 18, they'll then eat into the Big 12, whether indirectly (SEC) or directly (ACC). SEC poaches Clemson and Florida St from the ACC, and now the ACC is left with only 13 teams (including Notre Dame) so poach 5 from the Big 12 - say UCF, Houston, Cincy, West Virginia and Baylor. That leaves the Big 12 with 13 so Big 12 adds Wazzu, Oregon St, Boise, Memphis and SMU.

So now you have a Big 12 of Wazzu, Oregon St, Cal, Stanford, BYU, Utah, Arizona, Arizona St, Boise St, Colorado, Texas Tech, Oklahoma St, Kansas, Kansas St, Iowa St, Memphis, TCU, SMU. Which is fine....

But nowhere near an SEC with Alabama, Auburn, Georgia, Florida, Clemson, Florida St, LSU, Oklahoma, Texas, Texas A&M, Tennessee, Kentucky, South Carolina, Ole Miss, Miss St, Mizzou, Arkansas and Vandy.

Or even compared to a Big 10 with Ohio St, Michigan, Penn St, USC, Oregon, Mich St, UCLA, Washington, Nebraska, Wisconsin, Iowa, Indiana, Illinois, Purdue, Minnesota, Northwestern, Maryland, Rutgers

Scenario 2:
The PAC12 isn't raided and instead the B1G raids the ACC for 4 of Notre Dame, UNC, Virginia, Duke and/or Pitt. Smelling blood in the water, the SEC goes to 18 by taking Clemson and either Florida St or Miami. This leaves teams like Pitt, GaTech, Miami, Louisville, NC State as potential orphans that could be picked up by the Big 12.