I have seen many people say that one of the things that the arba minim (four species used on Succot) relate symbolically to the name of God, each species corresponding to another letter. What is the source of this idea?
The Zohar (Parshas Vayechi Daf 220) also writes that the four species correspond to the name of Hashem.
just an interesting side point: there's actually a hidden fifth "specie"
And it’s not just our awareness of nature that we need to focus on. Vayikra 23:40 says, "And you shall take yourselves on the first day the fruit of the Hader tree, of the date palm, the leaves of the hyssop and willow..." (close enough, anyway). Before we recite the blessing on these 4 minim- species most siddurs have a Y'he Rotsone, a brief prayer as kind of a preface to the mitsvah. In it we say, "...osiyos shimcha hamiyuchad tikarev echad el echad v'hoyu l'achadim b'yadi v'laidah aich shimchah nikrah alai..."- the letters of Your Name [Hashem’s four letter name] You should bring close, one to one, and unite them in my hand to know how Your Name is called upon me..."
The Arizal says that the 5 books of Moses parallel the FIVE elements of Hashem’s name; the last 4 books parallel the 4 letters and the first book parallels the kotes- the point of the inscribed letter yud! This mysterious kotes is created by the concave head of the yud. If the head of the yud is a right angle it is invalid. It’s disputed if the kotes is the top point or the bottom point of the head. Our point, if Hashem's 4 letter name is really 5 parts, then why not take 5 minim?
Rav Wolfson says the answer is already revealed in understanding the portion of the Y'he Ratsone quoted above, "...to know how Your Name [Hashem] is called upon me..." ME! That’s YOU!Hashem's Name is on the 4 minim and on us too! WE are the 5th specie paralleling the sublime kotes of the Yud!!
from Orchards of Jerusalem