A non-degenerate four-wave mixing technique has been applied to investigate carrier transport and recombination in heavily C-doped GaAs embedded in a double-heterostructure. The carriers were injected into the 1 µm-thick p-GaAs layer via the 50 nm-thick barrier of AlGaAs:C or InGaP:Si, using the light interference pattern of two picosecond laser pulses at 532 nm. The dependence of the nonequilibrium carrier grating decay time on the grating period allows the determination of minority carrier diffusion coefficients: D = 35 cm2/s for p-GaAs ($p_0 = 2 \times 10^{19}$ cm−3) with AlGaAs barriers and D = 27 cm2/s for p-GaAs ($p_{0} = 1 \times 10^{19}$ cm−3) with InGaP barriers. This increase of electron mobility at the higher doping level was found to be in agreement with the decreasing role of carrier-carrier scattering in heavily-doped p-GaAs. The fast recombination of nonequilibrium carriers in the vicinity of a front barrier layer was evident and more pronounced for an AlGaAs than for an InGaP barrier.