According to these links, the lack of friction and the heat from combustion in region 4 should be the giveaway:FannoFlow88 said:
Rayleigh flow is a type of compressible flow that occurs when a gas or fluid moves through a converging-diverging nozzle at supersonic speeds.
Region 4 is the throat of the converging-diverging nozzle, where the flow reaches its maximum velocity and minimum area. It is a critical point in the flow where the gas properties change significantly.
Region 4 can be identified by its characteristic shape in a pressure-enthalpy diagram, where it appears as a horizontal line connecting the two branches of the nozzle. It can also be identified by the sudden decrease in density and increase in velocity at the throat of the nozzle.
The giveaway that region 4 is Rayleigh flow is the presence of a shock wave at the throat of the nozzle. This shock wave is formed due to the sudden decrease in area and increase in velocity, causing a rapid increase in pressure and temperature.
Rayleigh flow has many applications in science and engineering, including in rocket propulsion, supersonic wind tunnels, and gas turbine engines. It is also used in the design and analysis of high-speed aircraft and spacecraft.