Explain the precise function of a four-quadrant converter (4QC) in a modern AC electric locomotive, particularly its role in active power factor correction and energy regeneration.

The precise function of a four-quadrant converter (4QC) in a modern AC electric locomotive is to serve as the sophisticated bidirectional power electronic interface between the locomotive's AC power supply (the catenary) and its internal DC traction link. This DC link then feeds the traction inverters which power the AC traction motors. The term "four-quadrant" refers to its capability to control both the magnitude and direction of active power and reactive power flow between the locomotive and the grid, encompassing forward and reverse power flow while allowing for leading or lagging current (though typically optimized for unity power factor).