We present data on the linear and nonlinear optical properties of the quaternary semiconductor AgGaGeS 4 having orthorhombic symmetry--a solid solution in the system Ag x Ga x Ge 1 - x S 2 with x=0.5. After numerical analysis of the phase-matching configurations for three-wave nonlinear interactions, first experimental results on optical parametric amplification producing tunable (3.8-11 μm) femtosecond idler pulses are described and the results compared with the calculations. Due to its higher damage threshold and greater variety of phase-matching schemes AgGaGeS 4 could become an alternative to the widely spread now AgGaS 2 in high power and specific applications.