First-principles calculations are launched to characterize the electronic, thermodynamic and vibrational properties of the ZnxCd1-xS0.25Se0.75 and ZnxCd1-xS0.75Se0.25 mixed crystals (x=0.0, 0.25, 0.50, 0.75 and 1.0). The equilibrium lattice constants and bulk moduli are deduced for the ten quaternary semiconductors. The electronic bandgaps and related electronic characteristics such as bowing are evaluated. Effect of pressure is seen, and the volume deformation potential and pressure coefficients are calculated. Strain energy calculations support suitability of this system to construct quantum devices. The vibrational frequencies at the centre of Brillouin zone are reported, and the entropic contribution in the free energy is found. Results are in good agreement with other data, wherever available.