The Four Seasons Tree is a large tree that the elves keep. The tree has different sections each with a different season. One side has snow, the other flowers, and another green leaves. It can be assumed that the last side of the tree has red and orange leaves like fall. When Sophie leapt her and Dex away from the Neverseen in book 1 this is where she landed. She used the tree as a landmark so Fitz would know where to find her, although it was Keefe that figured it out.
In Neverseen, the Four Seasons Tree is revealed to actually be the old gnomish leaders who the plague was originally released on.