The daled minim are compared to four of our most basic limbs. The Lulav is compared to the spine, the Esrog to the heart, the Haddas leaves are similar to an eye and the Arava leaves recall a pair of lips.
Chazal encapsulate the meaning of this inner symbolism with a quote by Dovid HaMelech –
[תהילים לה:י] "כל עצמותי תאמרנה ד' מי כמוך"
"All of my limbs will proclaim: 'Hashem, who can compare to you ?!' " [Tehillim 35:10]
That means that a certain aspect of taking the 4 minim is an exclamation of Hashem's greatness.
Consider that each one of the minim offers a unique way to declare Hashem's wonders and our fealty to Him.
When we stand tall and proud as His servants with a straight spine – we are shouting – "Hashem, who is your equal ?!"
When we look upon our fellow Jews with a kind eye, seeing the goodness inherent in each one – we are broadcasting – "Hashem, who can compare to you ?!"
When we use our lips to learn the wisdom of the Torah – we are intoning - "Hashem, who can measure up to you ?!"
And when we feel our hearts bursting with gratitude to the One Above for His kindnesses – we are pulsating – "Hashem, can anyone parallel you ?!"
Additionally, our sages teach us that the four minim are a comparison to four types of Jews. They must be taken together in order to fulfill the mitzvah. How can we increase the 'achdus', the unity between these four disparate brothers ? By realizing that each has a middah that he excels in.
The "lulav Jew" has a fierce Jewish pride – he will even take it to the maximum by mesirus nefesh al Kiddush Hashem.
The "haddasah Jew" sees the good in everyone, and sees Hashem's hand in all actions.
The "aravah Jew" always has a kind word to his fellow.
Finally, the "esrog Jew" is the whole package – motivated by a Jewish heart that beats with devotion to its creator. [ See Avos 2:9]