You are correct. The waitlist does not technically have a "Quartile 4", but instead goes in the order of High Priority, Quartile 1, Quartile 2, and Quartile 3, respectively. This also does not mean they have to select applicants in order. I think a lot of things go into their final decision on accepting applicants.
It is extremely interesting that they are interviewing the pre-interview hold folks this late in the cycle. I have not read or seen anything like this happening in past threads. Nevertheless, it is a privilege to even be considered.