Hi John,
First of all. You are using good monitor speakers in a (presumably) non-optimised room with no ambient sounds to speak of.
You are already miles ahead of any situation playing out live. PA speakers are nowhere near the quality of studio monitors (different purpose = different design/manufacturing choices) and the ambient noise levels go up to 80dB or more at live gigs.
The discussion about 1/4″ vs RCA is mute in a practice situation, provided you are not getting humms or other sound interference as a result of using an unbalanced cable. In a live/PA situation, if you are running cables from your controller to the main system, 1/4″ balanced cable is a must and RCA should be avoided (in general). For home use, if it sounds ‘unharmed’, I’d say don’t worry about it. We are into DJ-ing not audiophile listening.
The S4 interface is plenty good enough, imho. I would not add all the extra cabling, settings and such to insert an extra component in the audio chain. While in theory the FocusRite can be (and probably is) slightly better, you are not doing studio work, you are DJ-ing. Drop outs in Traktor can be eliminated by setting the latency different.
I would opt for the ease of use. Hook up your controller to your MBP, your speakers to the S4, plug in your headphones and done!