A Fourier-transient-grating technique is applied for the measurement of carrier diffusivity in low-doped n-type 4H-SiC epitaxial layers. It is shown that the diffusion coefficient can be extracted in a wide excitation range, 5x10(13) -3x10(17) cm(-3), which covers the transition region from the minority carrier diffusion to the ambipolar case. The minority hole and ambipolar diffusion coefficients have been determined: D-h = 2.35 +/- 0.4 cm(2)/s and D-a = 4.2 +/- 0.4 cm(2)/s, respectively. These values are 20% lower than the ones extracted from the Einstein relation using 4H-SiC Hall-mobility data presented in the literature.