The two by two focus is called the skim.
Like skimming through a book, or skimming the cream off the top, this view is how you
get the gist of what someone has to say without spending too much time with the details.
The skim only gives you access to the first four pictures of the first four galleries.
The skim starts with the preview images of the first four galleries. Click on a gallery, and it shows you a preview of the first four pictures of the gallery at the top of the page, with those four pictures listed one by one below. At the bottom of the page it shows you the preview images of the first four galleries again, so you can easily go from one gallery to another.
While the deep-dive shows you just nine pictures, the skim shows you as many as sixteen pictures, but it never shows you a whole gallery, so you never quite get the full story. But since users know that this is a common way to skim through the platform, they take special care to choose the first four pictures of the first four galleries wisely.
This also makes it so that the first four pictures of the first gallery are the only pictures which are accessible from every focus, so those four pictures are going to be what matters most to a person. Or at least, whatever is most important for them to show other strangers on the internet.