In this paper we consider domino tilings of bounded regions in dimension n 4. We define the twist of such a tiling, an elements of Z/(2), and prove that it is invariant under flips, a simple local move in the space of tilings.We investigate which regions D are regular, i.e. whenever two tilings t 0 and t 1 of D × [0, N ] have the same twist then t 0 and t 1 can be joined by a sequence of flips provided some extra vertical space is allowed. We prove that all boxes are regular except D = [0, 2] 3 . Furthermore, given a regular region D, we show that there exists a value M (depending only on D) such that if t 0 and t 1 are tilings of equal twist of D × [0, N ] then the corresponding tilings can be joined by a finite sequence of flips in D × [0, N + M ]. As a corollary we deduce that, for regular D and large N , the set of tilings of D × [0, N ] has two twin giant components under flips, one for each value of the twist.