The diameter 4 trees make up a certain class of genus zero coverings of the projective line with prescribed ramification only above 0, 1, and infinity. Over the complex numbers, the corresponding dessins d'enfants, i.e. the preimages of the real interval [0,1], look like graph theoretic diameter 4 trees. The ramification indices of the points above 0 are known Galois invariants called the "valency" of the tree, and moreover, the action of the absolute Galois group permutes the diameter 4 trees with a given valency. Thus the decomposition of the valency classes into Galois orbits is a natural question in the dessins d'enfants programs initiated by Grothendieck. By studying special models of diameter 4 trees in positive characteristic and then lifting them to characteristic zero, Zapponi shows that certain valency classes are entire Galois orbits.