We have performed non-degenerate four-wave mixing on free-standing porous Si samples, Within the framework of an idealized inhomogeneously broadened two-level system, values of 20 fs for the dephasing time are obtained for different porosities. Fast energy relaxation times are found in the range 1.5-3.5 ps for 64-73% porosities, respectively. They are attributed to carrier thermalization within the band tails, originating from porous silicon nanocrystallites and their surface states.