There are three form factors for FireWire devices.
The highest speeds (FireWire-800) are supported by the squarish, center-notched connector also known as the 9-pin connector. It has additional signals to allow balanced drivers (aka push-pull drivers) on the data lines to attain highest speeds. It can provide power for the external device.
The next highest speeds are provided by FireWire-400. It uses a squarish connector with one short side rounded, and can resemble the letter D in outline. It has 6-pins in the connector and the data cables. It can provide power for the external device.
The same FireWire-400 speeds are provided by the 4-pin connector. This is the one used in cameras and similar equipment. It simply drops the power pins, otherwise is identical ind function to FireWire-400. It is smaller and squarish, and has a center notch. It is often mistaken for "some kind of small USB".
Interoperability:
If the device can support FireWire-800, and the signals are present in ALL the cables, devices run at FireWire-800 speeds. Otherwise, they automatically run at FireWire-400 speeds. Devices that claim to "adapt" from one to another generally just provide the correct connectors and drop the extra signals required for FirWire-800. The devices themselves drop back to FieWire-400 and work just fine. This whole ecosystem is remarkably well behaved -- there are no stories of manufacturers producing devices that do not inter-operate.
additional information and pictures here:
https://en.wikipedia.org/wiki/IEEE_1394
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