The new S4 comes with a new look, better functionality and better intergration with IOS products. Its a great four channel controller that is nice and portable, and easy to pick up and use.
Traktor Kontrol S4 MK2 Review
The newer S4 comes with an updated look, that looks great, subtle changes in design like thinner fader caps and grey filter knobs, with RGB buttons and aluminum jogs give it a more sleeker look.
It comes with Traktor Pro 2 full version that is mapped for Traktor DJ.
The RGB buttons on the new S4 are used for transport and cue controls and they are much improved from the older model. They are more tactile and responsive. Something to note is the brightness of the RGB buttons, they are very dim when on USB power but when plugged into a power source they are decently brighter.
The jog wheels are aluminum topped with an aluminum finish to give it a nice modern and sleek look. If you scratch they feel and work a lot better than the previous version.
Booth output was added as well, it is a TRS output that can be switched between booth and main.
The newer S4 also gives you better control over the remix decks.
Traktor added a flux mode button by removing a tempo fader.
If you want better control over Traktor, the new S4 is the best controller if you want a plug and plug controller.
If you are thinking of upgrading for the MK1, its a little difficult to justify the extra money.
With better control of the remix deck and better IOS integration its a good mid range controller.
For another controller in this range, check out the Pioneer DDJ-SR to compare.