Why are some FireWire (1394a) cables 4-pin and others 6-pin?

The 6-pin cable carries data and draws power from the FireWire bus. The 4-pin cable is used to carry data only, it does not draw power from the FireWire bus. It's also called iLink. The diagram below is a brief summary of the differences in available FirewireDirect cables.

6-pin to 6-pin cable. This cable supplies power to the FireWire bus for devices like small hard drives that cam run on bus power. Most commonly Firewire/1394 devices such as FireWire Hard Drives, FireWire CDRW, DVD and other common 1394 computer peripherals use this kind of cable connector, and most computers and PCI cards have the same standard 6-pin connector. FirewireDirect products that ship with cables ship with these 'standard' cables unless otherwise indicated.

6-pin to 4-pin cable. This cable does NOT supply power to the FireWire bus and is usually used to connect to DV Cameras or other self powered devices to a standard 6-pin computer connection. Some Windows computers will have a 4pin 'I.Link' connector, especially on certain laptops, and if so you need a 6/4 cable to connect standard Firewire devices to this type of computer.

4pin-4pin cable. Used to connect 4pin iLink devices (almost always video cameras) to other iLink devices or to computers (Sony and a few other Windows computers) using the 4pin iLINK ports.