Investigations on ion transport mechanism in the new Ag+ ion conducting quaternary solid electrolyte systems: (1 − x)[0.75AgI: 0.25AgCl]: xKI, where 0 < x < 1 in mol. wt.%, are reported. The quaternary systems were prepared by solid solution reaction of an alternate host: `a quenched [0.75AgI: 0.25AgCl] mixed system/solid solution', instead of the traditional host salt AgI, and KI. The compositional dependent conductivity studies on the quaternary systems identified the composition: 0.7[0.75AgI: 0.25AgCl]: 0.3KI, as the Optimum Conducting Composition (OCC) having conductivity σ ~ 5.9 × 10−3 S cm−1 at 27 °C. For direct comparison of the room temperature conductivity behaviour of the quaternary OCC, AgI-based ternary solid solution composition: 0.8AgI: 0.2KI, resulting into the well-known superionic system: KAg4I5, has also been synthesized in the identical manner which exhibited σ ~ 1.12 × 10−2 S cm−1 at 27 °C. The conductivity of newly synthesized quaternary OCC was slightly lower than that of as prepared ternary KAg4I5. However, in the open ambient conditions $\sigma $ of OCC sample remained practically stable for over hundred hours while that of KAg4I5 decreased by more than two orders of magnitude in the same duration. The phase identification and material characterization studies on the quaternary OCC have been carried out using XRD and DTA techniques. The ion transport mechanism has been characterized on the basis of experimental studies on some basic ionic parameters viz. conductivity ($\sigma $), ionic mobility ($\mu $), mobile ion concentration (n), ionic transference number (tion) and ionic drift velocity (vd). Solid state batteries have been fabricated using the newly synthesized quaternary OCC as well as KAg4I5 as electrolytes, sandwiched between Ag/I2 electrode couple and the cell potential discharge performances have been studied under varying load conditions.