Isochronal annealing and thermal evolution of electron- and proton induced defects in semi-insulating 4H-SiC have been investigated by positron annihilation experiments. Positron lifetime and Doppler broadening measurements were performed to investigate the thermal stability of the radiation induced defects as well as possible clustering mechanisms during the isochronal annealing. The as-grown samples contain vacancy-type defects. These defects exhibit a positron Lifetime of about 248 ps yielding a mean lifetime of 142 ps. The observed lifetimes suggest complexes with divacancy character. The concentration of these defects is in the range of 3x10(16) cm(3). For the bulk lifetime a value of 138 ps follows. The mean lifetime shows an increasing dependence on the radiation dose. The grown-in defects are found stable even at 1600 degreesC. The annealing behaviour for the irradiated semi-insulating sample is quite different from that observed in n- or p-type SiC.