In the fourth part of the article, the application of the proposed method for the complete optical characterization of crystals to optically uniaxial crystals is considered. The object of the study was lithium niobate (LiNbO 3 ) crystals. Refining the orientation of the optical indicatrix in the samples under study, we confirmed the conclusion by direct measurements that for the implementation of the proposed combined ellipsometric method, knowledge of the crystallographic orientation of the crystal under study is completely optional. The obtained values of the optical constants of the undoped LiNbO 3 crystal [ n o = 2.280(±0.003), n e = 2.202(±0.002), Δ n = – 0.0775(±0.0015)] fully confirmed the correctness of the proposed method and its efficiency in the investigation of uniaxial crystals. The sensitivity of the combined ellipsometric method was tested on LiNbO 3 crystals subjected to high-temperature annealing in an H 2 O atmosphere. The optical constants obtained for an undoped LiNbO 3 crystal [ n o = 2.2455(±0.0015), n e = 2.1965(±0.0015), Δn = – 0.049(±0.001)] and a magnesium-doped LiNbO 3 crystal [ n o = 2.2445(±0.0015), n e = 2.1756(±0.0007), Δ n = – 0.069(±0.001)] is significantly less than the optical constants of the as grown crystal. It is assumed that the main reason that caused such significant changes is high-temperature annealing in an H 2 O atmosphere. Our main goal was to show the applicability of the method to the analysis of possible changes in the optical constants of a crystal caused by the influence of various factors on its properties. The results obtained show that the goal has been achieved. Key words : ellipsometry, optical indicatrix, principal refractive indexes, uniaxial crystals, lithium niobate.