The transformation of the vibrational spectrum of Cu₂Zn(Sn_1-xGe_x)S₄ single crystals over the entire composition range (0 ≤ x ≤ 1) is studied experimentally by low-temperature Raman scattering and photoluminescence spectroscopies, as well as theoretically in the framework of density functional theory (DFT). It is shown that unlike "classic" mixed binary II-VI and III-V compounds, which are characterized by either one- or two-mode behavior of spectra transformation upon composition variation, the vibrational modes of the quaternary semiconductor Cu₂Zn(Sn_1-xGe_x)S₄ exhibit both types of behavior within the same alloy system. DFT calculations reveal that the two-mode transformation is in fact observed for the vibrational modes, which possess a very small dispersion across the Brillouin zone, that is typical for a molecular crystal. These modes are due to the "breathing" motion of sulfur within GeS₄ and SnS₄ tetrahedra. The effects of structural (positional) disorder of mixed crystals are analyzed based on Raman scattering as well as photoluminescence results.