You pay when your application is complete, all addenda are attached, and you are ready for the app to be sent to the school (at which point, you cannot make changes to the original applications.)
To those who assumed the 4th section was the experimental, bear in mind that June 2022 was an anomaly. They normally don't tell you which one is the experimental. It can be either of the two of the repeated section.
I'm not in your situation yet, but I am facing a similar issue and was asking some people I know who work in admissions at a T14 school. Their guidance was the same as canihazJD... Don't start at a school you wouldn't be happy to graduate from. The …