Stoichiometric B4 thin films have formally divergent surface energies, which arise from the intrinsic dipole of the unit cell. Previous density functional theory studies have predicted that below a critical thickness this results in relaxation to the nonpolar planar h-MgO structure. The calculations presented here demonstrate that h-MgO-structured ZnO thin films are themselves unstable with respect to further relaxation to the d-BCT structure, which restores near-tetrahedral local coordination while minimizing the surface dipole. Although the B4→h-MgO relaxation is disfavored for slabs thicker than 20 layers, d-BCT is predicted to be the favored polymorph for slabs up to 54 layers. Nudged elastic band calculations and vibrational analysis indicate that the h-MgO→d-BCT relaxation is spontaneous at nonzero temperatures.