We applied four-wave mixing (FWM) technique for investigation of high temperature chemical vapour deposition (HTCVD) grown 4H-SiC samples with different doping levels. The determined minority electron and hole mobilities in heavily doped crystals at doping densities of 1019 cm(-3) were found to be equal to 116 and 52 cm(2)/Vs. In semi-insulating (SI) crystals, the ambipolar diffusion coefficient Da = 2.6 - 3.3 cm(2) A and carrier lifetimes of 1.5 - 2.5 ns have been measured. Irradiation of SI crystals by 6 MeV electrons resulted in essential decrease of carrier lifetime down to &SIM, 100 ps and clearly revealed the defect-assisted carrier generation with respect to two-photon interband transitions before irradiation.