I have two questions about the mitzvah of the four species.
1) The species involved are weird. One would expect that they include species common to the land of Israel, like grapes or olives. But none of the "seven species" appears in the four species. In fact, the four species have very little importance in Tanach in any context except for this mitzvah.
2) Chazal say that the four species represent four types of people who must be united for the mitzvah to be performed. What is the source for this midrash?
To answer these questions, we must look at where each of the species grows.
The lulav is part of the palm tree, which grows in the desert.
The etrog is a citrus fruit, which grows in the coastal plain (like the Jaffa orange).
The arava grows in valleys ("arvei nachal").
The hadas grow in mountainous regions.
We see that each of the species grows in a different part of the land of Israel. A person living in Ein Gedi, for example, would have plenty of palm trees around, but no etrog trees. Meanwhile, a person living in Kfar Sava would have plenty of etrogs but no lulavs. When everyone gathered to Jerusalem for Sukkot, the Kfar Sava people would have to swap their etrogs for the Ein Gedi people's lulavs, so that each group would have both lulavs and etrogs. Similarly, they would have to find people from the mountains (Hevron?) and valleys (Afula?) in order to obtain hadasim and aravot.
It seems that the four species is a mitzvah that no individual could perform on their own. Only one of the four species would grow in their hometown, and the others would have to be obtained from people living in other parts of Israel. Thus, it was a mitzvah that had to be performed by the entire people, working together, coming together to Jerusalem.
After the exile, Jews no longer lived in each part of the land of Israel, and they would not all gather to Jerusalem on the holidays. Thus, it no longer made sense to talk about the mitzvah as uniting people from different geographic regions. Instead, Chazal said that each species represented a person of a differing type of piety - and people of all such types can be found in any place.