What is the deeper significance of the mitzvah of the four species - the lulav, etrog, hadassim and aravot - and why does the Torah command us specifically to take them together on Sukkot?

The four minim (etrog, lulav, hadassim, aravot) symbolize the entire Jewish people, from tzaddikim with Torah and good deeds to those lacking both. HaShem commands their unity, teaching all Jews are essential for serving Him. Representing body parts (etrog-heart, lulav-spine, hadassim-eyes, aravot-mouth), the aravot-hitting on Hoshana Rabbah stresses controlling our speech – a primary source of suffering – and perfecting all aspects of ourselves. The lulav, created solely for its mitzvah, teaches HaShem fashioned the world for Torah. Shaking it with intention connects us to unseen spiritual effects, divine order, and Torah's perfection.