Unless the PAC12 can get the Big 12 down to 4 teams (not including the 4 new additions), then any Big 12 team would have to pay a big buyout and exit fee. Which makes it a non-starter.
So it has to be 4 legacy members.
After that, if they want to add more, they could go for Houston (or BYU), who I have long heard they do like.