XLR (4-Pin)
Direct Answer
4-pin XLR uses the same connector shell as standard 3-pin XLR but carries entirely different, incompatible signals depending on the equipment: intercom headsets, balanced headphone outputs, or 12V DC power -- with no universal standard or keying to prevent connecting the wrong device to the wrong port.
Source: De facto industry conventions (no single universal standard)
Specs
| Shell | Same as standard 3-Pin XLR (4 pins) |
| Intercom Headset Pinout | Pin 1: Mic Ground | Pin 2: Mic Signal | Pin 3: Headphone Return | Pin 4: Headphone Signal |
| Balanced Headphone Pinout (common convention) | Pin 1: Left (+) | Pin 2: Left (-) | Pin 3: Right (+) | Pin 4: Right (-) |
| DC Power Pinout (common convention) | Pin 1: Ground | Pin 4: +12V (nominal) | Pins 2 & 3: Unused |
One Shell, Three Unrelated Signal Types
Unlike 3-pin XLR, which has one universal pin assignment everywhere it's used, 4-pin XLR uses the same physical connector shell for at least three genuinely different, non-interchangeable purposes. Intercom headsets -- Clear-Com and Telex systems are documented examples -- use it for a combined microphone-and-headphone connection, with separate pin pairs for each direction. Some professional and audiophile headphone amplifiers use it for balanced stereo output, commonly wiring left and right channels as separate positive/negative pairs, though this specific pin order is a widely-documented convention rather than a certified universal standard. And many professional video cameras and location-audio equipment use 4-pin XLR purely for 12V DC power, with no audio signal at all.
Because the shell and pins are physically identical across all three uses and nothing keys or prevents cross-connection, plugging the wrong device into a 4-pin XLR port carries real risk -- a power connector into an audio input, for instance, could easily damage equipment on either end. Always confirm what a specific 4-pin XLR port is wired for before connecting anything to it.
A related but physically distinct connector, Mini-XLR (also called Tiny XLR or TA4, after its original Switchcraft part numbers), is smaller than full-size 4-pin XLR and commonly used on wireless microphone beltpacks and some studio headphones -- it is not interchangeable with full-size 4-pin XLR despite the similar name.