Vertical decomposition of arrangements of hyperplanes in four dimensions

We show that, for any collection ℋ ofn hyperplanes in ℜ4, the combinatorial complexity of thevertical decomposition of the arrangementA(ℋ) of ℋ isO(n4 logn). The proof relies on properties of superimposed convex subdivisions of 3-space, and we also derive some other results concerning them.