Four-pin XLR connectors are used in a variety of applications.
They are the standard connector for intercom headsets, such as systems made by ClearCom and Telex. Two pins are used for the mono headphone signal and two pins for the unbalanced microphone signal.
Another common use is for DC power connections for professional film and video cameras and related equipment. Some desk microphones with LEDs use them. The fourth pin is used to power the LED indicating that the microphone is on. Other uses for the four-pin XLR include some scrollers (color-changing devices for stage lighting), AMX analog lighting control (now obsolete) and some pyrotechnic equipment.