There are quite a few things to think about with these 2 products:
1: The S4 comes with a full version of Traktor. The Denon comes with an LE version of either Virtual DJ or Traktor depending on where you live. If you haven’t already got DJ software and want to use the Denon to its full potential, you’ll need to purchase software separately which then bumps up the price a bit ($299 for Virtual DJ, $89 for Virtual DJ)
2: Are you going to want to attach any other equipment to them? i.e. cd players/record decks/mp3 player? The Denon is a standalone mixer as well as a controller, so you can use it without a computer with just cd players etc. attached and it will function just like a regular mixer. With the S4 (i think) you can attach cd players etc. but they have to be routed through traktor which means you have to have a computer connected and turned on (can someone confirm this for me?) I think this also goes for connecting a mic as well.
3: Are you going to want to use Timecode with this now or at any point in the future? With the S4 you can purchase the timecode upgrade and use the same hardware, with the denon you can’t use timecode full stop with Traktor without purchasing an NI soundcard, and with Virtual DJ it can be done, but you’ll need at least another soundcard with 1 output, or ideally a 4 in 4 out soundcard.
4: Do you want to be able to plug and play, or do you want to fiddle around with mappings etc? With the S4, you can plug it in and it will just work, and the jog wheels will use the HID protocol for better control. If you want to change any of the controls you can switch it to MIDI mode and map it yourself, but then you lose the hi-res jog wheels. With the denon, it comes with pretty good traktor and virtual dj maps, but both can be fully customised to suit your needs.
I have the denon, and it’s a fantastic piece of kit. I use it with traktor, and the jog wheels are more than good enough resolution for what i do (I dont do any scratching or anything with them). The fact that it is a standalone mixer means that if anything happens with my pc, I can just play the mp3 player that i keep connected to it permanently while I sort out any computer related problems. Being a mobile dj, having extra inputs always comes in handy for various things as well.
Both have balanced outputs, the denon has 2 mic inputs whereas the S4 only has 1, so not really much difference there. I have never used an S4, so i can’t comment on it at all, but I highly recommend the Denon!
At the end of the day it depends on what you want out of the controllers and what you are going to use them for, both now and in the future.