Graphical Code Puzzle

One of the puzzles in my Ditch Day stack was a graphical code, designed by Caltech alumnus Adam Jermyn. The puzzle was originally hand-drawn on a series of slips of paper; each image on this page is one slip of paper, scanned. The final answer should be a four-digit number.

This final clue should resolve to a four-digit number:

The correct answer, encoded as ROT13, is GJBGUBHFNAQNAQFRIRAGLRVTUG.