If you want to create a new sub image all you have to do is drag you mouse in the scan area, maybe rotate and resize the resulting rectangle, then hit the A key or click on this icon:
The icon shows an image from the main scanner area being added into the column of sub images.
Note that you do not have to limit yourself to adding only 4 images from a scan of 4 photos. You can also save details from within the images, as shown in this screenshot:
