The four species subjugate the body to the soul

ולקחתם לכם כו' ושמחתם כו'.

"And you shall take for yourselves [the four species]... and you shall rejoice..." (Vayikra 23:40)

The Sefas Emes begins from the mitzvah of the arba minim (four species), which the Torah pairs directly with the command to rejoice on the chag.

ע"י המינים הללו מקבלים בנ"י פנימיות השמחה של החג.

Through these species, Bnei Yisrael receive the inner essence (penimiyus) of the simchah of the chag.

Taking the four species is the vehicle through which Klal Yisrael draws in the deep, inner joy of Sukkos.

כמ"ש תודיעני אורח חיים שובע שמחות את פניך.

As it is written: "You will make known to me the path of life; abundant joys are in Your presence." (Tehillim 16:11)

He brings the passuk that speaks of being sated with joy in Hashem's presence, hinting at a fullness of simchah that comes from closeness to Him.

פי' להיות נשבע מן פנימיות השמחה.

The meaning is: to become sated from the inner essence of the simchah.

"Abundant joys" (sova semachos) means being filled and satisfied by the inner penimiyus of joy, not merely a surface gladness.

ולהיות נשאר מן השמחה על כל השנה.

And so that some of this simchah should remain for the entire year.

The joy absorbed on the chag is meant to leave a lasting residue that carries a person through the whole year.

כמ"ש נעימות בימינך נצח להיות דבר של קיימא.

As it is written: "delights at Your right hand forever (netzach)" (Tehillim 16:11) — that it should be something enduring.

The continuation of the same passuk, "netzach" (eternity), teaches that this simchah is meant to be a permanent, lasting acquisition.

וגם לשעבד כל הגוף אל הנשמה כדאיתא במד' כל עצמותי תאמרנה.

And also to subjugate the entire body to the neshamah, as it is brought in the Midrash on "All my bones shall say [Hashem, who is like You]." (Tehillim 35:10)

A further purpose of the four species is to bring the whole body into submission to the soul, so that every limb participates in serving Hashem, as in "all my bones shall say."

אתרוג דומה ללב לולב לשדרה הדס לעין ערבה לפה.

The esrog resembles the heart, the lulav the spine, the hadas the eye, and the aravah the mouth.

Chazal compare each of the four species to a part of the body — heart, spine, eye, and mouth — the very organs through which a person can either serve Hashem or stray.

היינו לשעבד כל אלה האברים אל הקב"ה.

That is, to subjugate all these limbs to Hashem.

By taking the species that correspond to these limbs, a person dedicates and subjugates the heart, spine, eyes, and mouth to the service of Hashem.

ואלה המינים מסייעים לזה.

And these species assist in this.

The mitzvah of the arba minim is itself a help and a means toward bringing the body under the rule of the neshamah.

וזה עצמו הרמז דנצחין דינא.

And this itself is the hint of "winning the judgment" (natzchin dina).

This explains the custom that taking the four species signifies that Bnei Yisrael have "won" the din of the Yamim Noraim — a victory rooted in the body's subjugation to the soul.

כי כל המלחמה עם הסט"א שרוצה להפריד הגוף מן הנשמה.

For the entire war with the Sitra Achra (the "other side") is that it wishes to separate the body from the neshamah.

The whole battle against the forces of impurity is their attempt to pull the body away from the soul, severing the limbs from their inner spiritual source.

ובנ"י מנצחין לשעבד הגוף אל הנשמה:

And Bnei Yisrael are victorious in subjugating the body to the neshamah.

By taking the four species and binding the body's organs to the service of Hashem, Klal Yisrael triumphs over the Sitra Achra — this is the true "winning of the judgment."

Summary: Through the mitzvah of the arba minim, Bnei Yisrael draw in the penimiyus of the simchah of Sukkos — an abundant, inner joy that is meant to endure for the whole year. The four species correspond to the heart, spine, eye, and mouth, and taking them subjugates the entire body to the neshamah. This is the deeper meaning of "winning the judgment": the whole battle against the Sitra Achra is its attempt to separate body from soul, and by binding the body's organs to Hashem's service, Klal Yisrael is victorious.