I'll consider you know the basics of DJing and how soft/hardware works in these cases so let's try to split this question:
Xponent VS Traktor S4
The main advantage of the XPONENT is that IT FREAKIN' BLINKS! It has an awesome club presence, just check any youtube video to get that.
The main disadvantage is that it is a piece of crap! Excuse my french but there was a MASSIVE set of Xponents that had cheap ass electric board with cold solders and it would break easily. It seems to be fixed, but I discovered that M-Audio/Avid support is TERRIBLE! So terible that I'm not buying anything from them ever.
Main advantage of the S4: 2 stereo outputs AND a headphone output, while the xponent only has a single stereo out and a headphone output. Oh yeah, it also has a MIC INPUT that Xponent doesn't have.
Mais disadvantage: The jog wheels are small. Really small. I mean, have you tried it? It makes me feel... I dunno, I fell like I'm using an old Behringer or something.
The number of buttons and knobs are almost the same !BUT! if you are into the whole custom scene, the Xponent has a little feature that makes it moar awesome. There's a 1-2 switch in the front that acts as a second controller.
So when you change the switch from 1 to 2 you can then REMAP everything on the controller.
Torq 2 vs Traktor Pro 2
I'm not even talking about Torq 1 since that's no longer supported. Let's talk about Torq 2 vs Traktor Pro 2, it has only a few differences:
Torq 2 has a special crossfade option that adds some effects, it's a cool concept: you can mix 2 songs using a filter, so as you move from one song to the other the filter opens/closes. Their website has a couple of videos on this.
The auto synch across 4 decks feels more right than Traktor, and it works better. This is relevant if you like to change drastically BPMs between songs.
AND torq2 has INCREDIBLE VST SUPPORT. EVEN if the VST crashes the whole software keeps running.
Traktor is 100% fully customizable. I mean, you can map buttons to change to the library function to select tracks without touching your laptop. If you're into DIY custom midi mapping, this is heaven.
Also, Traktor has multiple EQ choices, Torq2 only has one, so instead of having a Bass/Mid/Tremb eq you can use a Bass/MidBass/MidTremb/Tremb eq.
To be completely honest, I don't have a lot of experience with the S4, I own a Xponent with Torq 2 - (heck I even did some lectures on using Torq and exploring it's functions) - and I would NOT recommend using it. I had a whole lot of problems with support - Torq sometimes decides that it just won't work - the application does not open under any circumstance - and all the seven times this has happened, the only solution was to format my computer. I tried using a Windows XP SP3, Windows XP SP2, Windows 7 32 bits and windows 7 64 Bits. I had problems with all of them.
So, long story short: Avoid M-Audio/Avid for the moment, go for Native Instruments.
BTW: I had some experience with Traktor Pro (I used it for about 6 months), I personally don't like its controls and GUI interface, it fells wrong. But the audio quality, response and amount of customization is beyond awesome.