Line 7 - In four scenes is the key hidden in the unbalanced divide of their number is its shape defined.

As many others, I have taken this to mean that the key is hidden in four of the images in the book.  
The unbalanced divide of their number (the number of the four scenes) would be 3 to 1.  

There are of course 7 images in the book.  
4 on two pages, back to back, and 3 that appear later in the book.  

One possibility is that four of these individual images contain the hidden key and that 3 of these images show how its shape is defined.  

(I am also tempted to consider that the text that appears later, "three shepherds works reveal the meaning of a mortals life and each shall describe the edifice of a tomb from above beside and below"  alludes to this).  

But what this doesn't tell us is what the last of the four images is for.  

It could mean that the fourth image shows the completed key and that the first 3 images show how to construct it.  Based upon things I've found in the first 3 images of the book, it might support this approach.

However (and not as an attempt to confuse), there is the possibility that "scenes" refers not merely to singular images but to composite images.  

Several who have held the book pages to the light have noticed how the back-to-back images relate to each other, to produce a composite image.  

So when the book speaks of "4 scenes", it could be referring to these composite images making one "scene".