An easier way to play real vinyl if you have the right cables and the MC has line/phono switches and a way to incorporate 2 more channels of audio sources from something other than traktor. Plus any features the mc6k has that traktor's internal mixer doesn't.
Keep in mind that you'll be going down To 2 channels wit this setup. Traktor can't use 2 sound cards and vinyl control only works with an NI interface or scratch certified mixer. So if you want to run more than 2 decks from the software (including sample decks) you can't do it with an audio 4 unless it's in internal mode.