Well if you want to go Traktor our advise is to go NI controller. So that is kinda limiting your choice. If you want 4-channel and jogs then the S4 (mk2) is pretty much your only option. So choices are a tad limited.
Be aware that Traktor and the iPad Traktor are two different environments. I believe for a short time after release they would sync, but that got stopped after a while. Not sure if that function ever came back.
If you have to maintain two platforms and/or need to get used to a new one (albeit with the same name), you might consider the other options that seem a better fit for iPads and have a bit broader controller support.
That said, Not sure any iPad-centric controls out there have 4 channels and standalone functionality, not even the Mixon 4 by Reloop.
On-topic: The S4 is – to the best of my knowledge – NOT a standalone mixer. So all audio has to go through the software, in which case the question is if the iOS version of Traktor can handle 4-decks. And the answer to that question is no I believe. External/Line inputs are only supported by the full PC version of Traktor.
Hopefully some of our more Traktor-oriented readers can confirm this or set me straight.
.
