We calculate positron annihilation parameters (lifetime and high momentum part of the Doppler broadening of the annihilation radiation) for different native defects in 4H-SiC. To figure out the influence of lattice relaxations, we consider atomic coordinates in the ideal lattice as well as those determined as minimum energy structures from ab-initio calculations. We then compare the calculated annihilation parameters to experimental data on irradiated bulk SiC, where vacancies in different sublattices can be identified. If we qualitatively take into account the influence of the trapped positron on the relaxation pattern, we find good agreement with experimental data.