Letters from Books - start by opening one hint at a time to see if it's enough information
If you haven't done it yet, cut out the red rectangle with all the letters. Now, you need to spine it around something. But what? Have you seen the shape at top somewhere else?
Try twisting the strip of paper around the pen, holding the symbol on the paper directly underneath the colored part of the clicker on the pen.
If you hold and have aligned the strip of paper properly, four letters will appear from the clicker and downwards on the pen. Those letters are F-O-U-R.
The right number and third digit in the code for the USB-stick is therefore 4