Four is the count where the arrangement changes the answer. Four quadrants, four panels in a row and four stacked strips are all four pieces, and they get used for completely different things.
The default here is 2 by 2. A 4000 × 3000 photo becomes four 2000 × 1500 px quadrants, each with the same aspect ratio as the original. That is the shape you want for a four-sheet print, for splitting a scan that holds four photos, or for a quartered texture.
One by four is the carousel and banner shape. The same 4000 × 3000 photo cut into a row gives four 1000 × 3000 px panels, tall and narrow, which is what a swipeable post wants. Four by one gives four wide strips, which is how a long infographic gets broken up.
The cut lines are draggable in both directions, so the four pieces do not have to be identical. Move the vertical line to keep two people in separate quadrants. Move the horizontal one so a caption stays with the picture it belongs to. Reset to equal is one click away when you overdo it.
Everything is processed by the Canvas API inside this page, so your file never leaves your device, there is no queue, and no watermark ends up on the output.