Hi,
Powered devices require 6-6 (the extra two pins are for powering devices). Laptops generally have 4 pin O/P due to the fact that it's smaller and not using the battery to power an external device extends battery life (iLink on Sony for example).
Some external devices with their own DC power still use 6 pins! (Yamaha FX1 CD curner for example).
So, you can see it really depends on what O/P you have to hand and what exactly you're connecting to. There are no hard and fast rules.
Good luck,
Andy