4-pin digital audio pinout?

I just got the Pioneer DVR-106 drive, and I have a bit of a problem with
it. The drive has an analog audio connector and a digital audio
connector (at least that's what it says on the back of hte drive,
stamped out right above the connector). The manual only mentions the
analog, and even has a picture of the rear panel, with only one
connector drawn.
The problem is, the digital audio is a 4-pin connector. My previous DVD
drive had a 2-pin connector, by sound card has 2-pin internal digital in
(S/PDIF) connector, and the cable has two wires as well. I could not
find any information on the pinout of the 4-pin digital audio connector,
the best I could do is find a 4-pin to 4-pin digital audio cable, one of
the middle pins is not used (and I have no idea what the third wire is
for, signal, ground, and...?)

Anyone knows the pinout of this 4-pin digital audio?
Thanks,
--
Fedor G. Pikus
http://www.wild-light.com