Yes, I have used a 4pin cable on the 7pin ATI s-video jacks. You are correct, the extra pins are for the composite signal (I actually did a continuity test on the included composite adapter once to see for myself).
If you are having trouble getting the s-video output to turn on, you can try plugging everything in when the computer is off and then turning the PC on. It should detect all of the displays when windows boots. If that doesn't work, you can try another cable or another TV set.