What is the difference between two-pipe and four-pipe fan coil units in air to water heat pump systems?

In two-pipe fan coil systems can be applied only cooling or only heating in a building; they have 1 supply and 1 return and in the supply pipe flows either cold or hot water, according to the time of the year. On the contrary, in four-pipe fan coil systems there is the ability some rooms of a building to be heated and others to be cooled (depending on the orientation and the needs of each area of the building). These fan coils have 2 supply and 2 return pipes that allow hot or cold water to enter the unit at any time.