We show that the simplified bond-hyperpolarizability model (SBHM), previously used to accurately describe anisotropies observed in second-harmonic generation from Si-dielectric interfaces, also describes anisotropies observed in fourth-harmonic generation (FHG). FHG data from (001)Si-SiO2 interfaces show two contributions: one from the intrinsic response of the interface bonds and the second from roughness. SBHM calculations yield excellent agreement with scaled ps and ss polarization data and the phases of the pp and sp equivalents with no adjustable parameters. The SHBM also precisely reproduces the anisotropy data for samples with systematic variations of interface roughness, and predicts relative amplitudes consistent with measured fields to factors of about 2. (C) 2003 American Vacuum Society.