In a closed container, all coolants produce vapor pressure. For coolants comprised of a mixture of more than one liquid, the combined vapor pressure is a result of the sum of the partial pressures of each component. The pressure built-up in an engine's cooling system is a function of temperature and the relative amounts of component liquids in the coolant.
Adding ethylene glycol to water is an excellent way to reduce the freeze point of the coolant while modestly increasing the coolant's boiling point. A concentration of 50% (vol) ethylene glycol in water is a good compromise between improved freeze & boiling points and reduced heat transfer ability. The following table (based on calculated partial pressures with Antoine constants) compares the boiling points of 50% (vol) ethylene glycol and 100% water for various pressures:
50% Ethylene Glycol
According to Stant's FAQ, a thermostat typically open at ±2°F of its rated temperature and reaches full open at 15 - 20°F above its rated temperature. Using a 195°F thermostat, for example, means that it should be fully open around 210 - 215°F. A 4 lb pressure cap raises the boiling point of the coolant to 224°F with 100% water or 239°F with 50% ethylene glycol.
Using a higher pressure cap will prevent the engine from boiling over at higher temperatures but the higher pressure will unnecessarily stress the cooling system IF the engine overheats. A higher pressure radiator cap will NOT cause the engine to run cooler.
Be very careful about using a pressure cap that has a higher pressure rating than that recommended by the vehicle manufacturer.