I would say the S4… I am mixing on a two channel mixer, rapidly I wanted to add control over the other two decks in Traktor. I rarely play more than three decks at a time, with my 2 channel controller mapped to 4 deck control using extra buttons, but having the fader for volume there would be great. The added ability to upgrade it to scratch should you want to mess with TT/CDJ later is nice, which is probably in the future for me too. It really depends on your desires, but the S4 you probably won’t be upgrading later, wishing something else was there.
i always say this “get the best of anything your money can afford”.
its always better to have room for advance then be limited to.
There are a lot more S2’s changing hands just going by the threads in the Buy/Sell forum.
Why waste your time learning on something you will never use in the long run?
I dj out a shitload in Berlin… NEVER seen one S2 or S4… TOO BIG and not really built to move around and fit in the often real world club booth.
I take a pimped up Vestax VCI 100… if space is low… I have Faderfox DJ2 and DX2 in the bag and off we go… happy mixing.
Been using the faderoxes since 2004… not one fuck up, handmade and professional.