In the procedure prescribed for the first stage of a metzora's purification (Lev. 14:1-7), the Torah lists four items to be dipped into a mixture of blood and water: a live bird, a piece of cedar wood, a strip of crimson wool and a sprig of hyssop.
Rashi (to 14:6, citing Sifra) points out that while all four items are dipped, only three of them are attached together: the crimson wool is used to tie the wood and the hyssop, while the bird is just held along with them. And Negaim 14:1 adds that the tips of the bird's wings and tail are to be touching the bundle.
This sounds reminiscent of what we do when taking the Four Species on Sukkos; there too, three of them are bound together, with the esrog remaining apart but touching them. Does anyone suggest any reason for this commonality?