Perhaps I should explain the, erm... joke
It's based on a particular UK dialect so I doubt if many would understand it.
So, the joke goes,
what is the difference between a sock and a camera,
a sock takes five toes, whereas a camera only takes photos.
In this dialect, "toes" and "tos" (from the end of photos) sound similar. Also, "four" and "pho" also sound similar.
Hence, four toes, for photos.