Male 3-pin to female 4-pin.  6-inch, black.   

Many RGB controllers such as MeshTek BLE & Minleon NDB's have standard 3-pin, female output ports.  This 4-Wire Adapter allow these controllers to drive 4-pin, Double-Data line RGB/W+2 strings & fixtures.

WEC3 controllers have both 3-pin and 4-pin female light & power connectors, so these generally are not necessary.