The nature of defects in 4H SiC was studied by means of low temperature photoluminescence before and after energy-controlled electron irradiation. Analysis of experimental data from irradiation at energies above and below the Si displacement energy together with subsequent annealing leads to the conclusion that Si Frenkel defects have been detected experimentally in this material. Reasons why these are formed in some cases rather than carbon vacancy carbon antisite pairs are explored.