The Four Species, Alef, Hashem, Man, and The Tzaddik

הלכות שבת ז:נה

("The Power of Aleph" by David Friedman)

The arba minim (four species) correspond to the letter א.  The myrtle and willow hint to the letters י and  ה (of Hashem's 4 lettered name), which are the "higher point", and the esrog parallels the final ה, which relates to both the "lower point" and to malchus.  The lulav parallels the letter  ו, which is the ו inside the א that binds the two points (the higher and lower).  Through the completeness of the letter א, the concept of "man (אדם)" is then achieved, for "there is no man without an alef."  Thus, the secret of man is included in the four species.
What one hopes to achieve through his service of Hashem is to draw down the splendor and beauty of the esrog, which corresponds to the final ה.  However, when it comes to the "higher point," which corresponds to Moshe and the true tzaddik, it is impossible to see such splendor and beauty , for we have no concept of what it could possibly be.  This is because the "face of Moshe is like the face of the sun," and at this time it is impossible to look at it.