The Four Seasons Tree is a large tree in the elven world. The tree has four different sections, each with a different season. One side has snow, the other flowers, and another has green leaves, as Sophie described in the book. It can be assumed that the latter side of the tree has red and orange leaves like autumn.
When Sophie and Dex light leaped away from the Neverseen in Book 1: Keeper of the Lost Cities with the lantern on the bridge, this is where she landed. She used the tree as a landmark so Fitz would know where to find her. However, Fitz was in denial of her still being alive, and thought it was his imagination, so it was Keefe who figured it out and convinced Fitz to find her.
In Neverseen, the Four Seasons Tree is revealed to actually be the old gnomish leaders who the plague was originally released on. Similar to a Wanderling, deceased gnomes are reborn as plants. Not all trees or plants are gnomes, but generally the most spectacular are. This is an example of how four gnomes combined to forge one tree.