It woud be arrow 4. How I found this is by imagining that arrow 4 was connected to the corner of A'. Mentally I then had corner A' follow the arrow, taking the whole shape with it. Once it finished following the arrow you can see that A' and the other angles match up with ABCD. I added an image where I tried to demonstrate it, but it's a bit hard to explain.
Hope this makes sense! Let me know if you need any clarification
