By most accounts, the S4 isn't actually a good bit of kit. Apparently a LOT of problems with them. However, it is one of the few controllers out there that has seperate mixer inputs, so the vinyl can still be played, and with timecode vinyl, you can still use the decks to control your mp3's. I did consider this route.
I also considered the Pioneer XDJ-Aero. Thought it was just a gimmick at first, but it is a nice piece of kit. Again, you can plug in your turntables, so can still play vinyl. And is wifi, so you can stream tunes to it over its own network. Overall a tidy piece of kit. And fully Rekordbox compliant, so another win there. And isn't actually a software controller. You can use it without a PC at all.
However, the mixer section was a bit basic for me, and with no FX send/return, it wouldn't be expandable how I would have liked. My DB4 is also missing send/returns, but has 50+ fx onboard anyway, so happy days.
530d M-Sport Touring.