Four-way solenoid valves are used chiefly on heat pumps to switch between heating and cooling modes:

Four-way solenoid valves are integral components in heat pump systems, primarily because of their ability to reverse the flow of refrigerant, allowing the system to switch between heating and cooling modes. This functionality is essential for heat pumps, which operate in both cycles depending on the season—extracting heat from the outside air in the winter and dispersing heat indoors, and vice versa in the summer.

In contrast, the other options suggest limitations of use based on system types or affirmations that do not recognize the universal applicability of four-way solenoid valves across all heat pump systems, whether residential or commercial. The versatility in seamlessly facilitating both heating and cooling operations makes true the statement about the primary application of four-way solenoid valves in heat pumps.