Well the S2 is obviously aimed only at Traktor users and has the tightest integration, but it also is all plastic (although decent plastic) and has the worst jogwheels (subjective opinion).
The mixtrack is an easy and overall good unit. It has no bells and whistles, but it is quite OK to start out with. You will want to upgrade at some time from the mixtrack, but reselling the mixtrack is usually not difficult as they sell like hotcakes.
The Denon is feature wise not a huge step up from the mixtrack (it has some kinks that are really cool though), it has good jogs for their size and most of all is built like a tank. The unit feels like it can withstand a tactical nuke, but again you will want to upgrade at some time unless you go down the road of minimal mobile setup DJ, where the Denon shines.
The N4 is again a plastic unit, but has LOTS of features going on there. It is more geared towards Serato but the Traktor mapping is quite good. It is the only 4 channel controller on your list and has lots of knobs etc. so you do not need to use the shift key all the time. All in all a good unit. If you want basically the same unit, but with an all metal construction you would have to go for the Reloop Terminal Mix series.
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