[en] We have measured polarization-dependent x-ray-absorption spectra (XAS) at the Cu K edge in magnetically-aligned powder samples of La₂CuO₄, Nd₂CuO₄, Bi₂CuO₄, Ca_0.85Sr_0.15CuO₂, and SrCuO₂, all of which contain Cu coordinated by four coplanar O nearest neighbors. We have also measured Cu 2p_3/2 x-ray photoelectron spectra (XPS) for all except the last compound. The measurements have been analyzed in terms of unrestricted Hartree-Fock calculations for CuO₆^10- and CuO₁₂^22- clusters, with energies and Coulomb parameters, including interactions with the core hole, for the Cu 3d_x²-y², 4s, and 4p orbitals determined in an ab initio fashion. It is well known that the intensity ratio of the two XPS peaks is sensitive to the degree of hybridization between Cu 3d_x²-y² and O 2p_σ orbitals in the ground state. We show both by empirical correlations and model calculations that the relative shift of the continuum absorption threshold for ε parallel z (where z is perpendicular to the plane of the CuO₄ units) and the shift of the main (3d¹⁰L) XPS peak are also useful measures of the covalency. The Cu 4p_z-like quasibound state observed in XAS is sensitive as well to 4p_z--O 2p hybridization. In the cluster calculations, good agreement with the experimental features is obtained with use of a single adjustable parameter, the energy of the O 2p_σ level (measured relative to Cu 3d_x²-y²). We show that this energy varies linearly with the difference in Madelung energies for the O and Cu sites