Stirling engine research has increased and opened new industrial opportunities. Stirling engines have the advantage of running on any sort of thermal energy. However, their biggest downside is their low experimental efficiency compared to Carnot's optimal efficiency. Thus, much research studies the optimal working parameters of this engine. Most existing models evaluate the Stirling engine's performance using the perfect gas assumption. However, this assumption is only valid for low pressures. At high pressure, real gas behaves differently from perfect gas. Hence the necessity and the interest of the conducted research work. The major goal of this numerical study was to analyze the influence of gas qualities on Stirling engine power and efficiency using a semi-real gas assumption. The impact of operating and geometric parameters on the performance of a four-cylinder Stirling engine (FCSE) were explored for three monatomic gases (Helium, Neon, and Argon) and one diatomic gas (Nitrogen). The dependence of volume ratio, regenerator form factor, load pressure, hot end temperature and rotation speed on gas properties were investigated. Major significant losses were accounted for at different conditions. To figure out parameters that maintain the highest performance, four different regenerator porosity (60%, 70%, 80%, 90%), three different cylinder diameters (3.5, 4, 4.5 cm) and three different phase shifts between the two pistons ( , , ) were evaluated. As regards, the obtained results, not only the optimum parameters that provide higher output power were identified but also predicted correlations for each studied filling fluid as a function of the geometric and operating parameters. The outcomes of the study would be very helpful for a new more powerful engine design. The obtained results clearly demonstrated that: (a) The choice of the filling gas is determined by the regenerator form factor. (b) The optimum volume ratio for nitrogen is roughly 1.1, whereas for Helium it is around 0.7. (c) For Helium and Neon, the output power diminishes as the volume ratio rises. (d) The output power increases significantly when the regenerator form factor (length by diameter) <0.2 and then reaches an optimum of around 2.1 kW for the Helium case. (e) Among the four studied gases, the use of Helium allows obtaining the highest output power. On the other hand, it is observed that the lowest output power is obtained when using Argon. Inside Stirling engines, the use of lightweight monoatomic gases (with high thermal conductivity and heat capacity values) boosts the output power. There is a variation in power up to 50% depending on the properties of the filling gas.