I've not really worked any vinyl emulation antics on the Zero 4 for a while but from what I can recall the analogue audio signal pickup from your time code vinyls shouldn't really affect the audio output of the actual digital audio file output from TSP (otherwise from the internal audio card output being used by TSP) as they're routed from a completely different set of channels with no analogue pass-through from the timecode vinyl. If there was any signal bleed/pass through from incorrect audio routings of your turntables you'd most likely be getting the high pitched whistling/squeeling sound of the vinyl time code signal and not necessarily record crackle. 

I never actually used TSP, however the DVS (DJdecks) I used didn't produce any hsss that I could detect.

I've not been able to reproduce the hsss/light crackle sound whilst using the Zero 4's internal with something like Ableton either. 


 
It's highly unlikely that this will be the cause but one source could potentially be from the main built in FX section. Double check to see if the X Y controls are completely all the way off (as far anti-clockwise as they will go) and set the step click rotary pot for the main FX switch all the way to bypass.
  
If its not that it might be to do with your latency timings. Although the audio output might appear to be stable from the main body of sound that is actually getting through, the buffer streaming blocks might be too small. Try increasing the latency timings within the Korg control panel. I reckon anything below 7ms is acceptable for the ASIO buffer streaming settings for vinyl control purposes. (with a setting of 3.5ms on the stream buffer depth setting). It will certainly feel a lot more responsive then other pro vinyl emulation solutions at that speed. However I reckon you might be able to get it down to 4 to 5ms ASIO buffer stream in combination with 2 to 2.5 stream buffer depth respectively.     

If you're running it at 96KHz but really want a faster stable ASIO streaming rate you might want to consider dropping down to 48KHz. Dropping down from 48 KHz to 44.1KHz will also allow you to tweak the ASIO streaming rate a little further for slightly faster speeds. However this doesn't apply to all computer and Zero 4 hardware combinations, there will be instances where the 96 and 48 KHz or 48 to 44.1KHz stable streaming rate threshold (i.e. without audio crackle. pops or hssss) will be the same. It really depends on the characteristic slights of imperfections within your hardware and general setup beyond a certain standard threshold.


BUT!!!!

Having said all that it might even be something that has been artificially added in by the software to sound like the crackle and hsss of vinyl, and if that is the case it sounds a little too artificial to me lacking the so called claimed "warmth" of vinyl. It sound too cut and shut and a little unatural to my ears.


 
If you really want to be certain, try the mixers sound card in ASIO mode with a different DAW application like Ableton. Or even just try playing an audio track in itunes or media player through the mixer audio card in none ASIO mode. If the hsss and crackle remains its your mixer that needs looking at in some other way, if it goes away then you know its the Traktor Scratch Pro software and mixer audio card combo that's causing it or you need to increase the latency timings if it isn't actually the mixer and TSP software that is intentionally designed to cause it. 

If after all that the unwanted hsss is still there, then maybe trying to update to the latest 1.60 firmware on the mixer might work. Full instructions for this process come included with the download from the official Korg support pages in PDF format which you can get direct from the following links...


Official UK Korg Zero 4 support pages:
 
http://www.korg.co.uk/products/discontinued/zero/dj_zero4.asp 

 
Official world Korg Zero 4 support pages:

  http://www.korg.com/SupportResults.aspx?productid=281