We applied picosecond four-wave mixing technique to investigate carrier diffusion and recombination in n-type 4H-SiC epilayers. The dependence of bipolar diffusion coefficient D on photocarrier density was measured in range from ∼ 10(17) to 10(20) Cm-3. We determined a decrease of D value from 3.4 to 2.2 cm(2)/S with increase of the photoexcitation level in range from ∼ 10(17) to ∼ 10(19) cm(-3), and found its increase up to 3.8 cm(2)/s at carrier density above 1020 cm(-3). Auger recombination governed decrease of carrier lifetime from 11 ns at ∼ 10(17) cm(-3) to 1.8 ns at ∼ 10(20) cm(3) has also been observed.