The diffusion coefficient of free carriers in 4H-SiC is experimentally determined over a large injection range. For this purpose two transient gratings techniques are utilized to detect light-induced free-carrier diffusive motion employing the changes in the absorption coefficient or in the refractive index, respectively. At high-injections a smooth reduction of the ambipolar diffusivity is observed with a following sharp increase above 3.10(18) cm(-3). Theoretical calculations based on dynamic band-gap narrowing in the former case and degenerated statistics in the latter case yields a reasonable agreement with the experimental data.