Technically the biggest moment to affect your sound quality is when you go from digital to analogue or the other way around. Multiple rounds of AD/DA conversion don’t help sound quality. Since you have only one round of DA conversion (digital in your laptop/DJ software to the analogue master output of your controller), assuming a decent quality of mixer, the analogue-analogue signal path should not harm your sound.
While you can do things to improve a bad source signal, the best thing is to have a good source. The sound card in the S4 is considered pretty good. So you are safe there. I would not hesitate to use what’s there (unless I am looking at Behringer, Skytec or no-name mixers) to plug in the easiest way.
Since you won’t use the mixer other than setting the volume with the channel fader, there is fairly little to gain by “getting to know” your own gear. Advice is to only use volume control on the external mixer and not use the EQ, set it all to 12 o’clock and have LPF and such off.
If you want to use a microphone a lot and your controller doesn’t support that in a way that suits your workflow (I have that problem now with my SX as the mic shares a channel and a very small and unpractical switch on the front with line 3), then it could pay to have a small mixer of your own. In that setup you could also hook your backup iDevice up.
So, there are reasons to buy your own mixer, like mic hook up, backup device and such. Sound quality and “getting to know” are not among them though imho.
.
