Regarding cables:
RCA is fine if you are going 2m or less (some might argue 5m is also OK) and if you go straight into a mixer that has a good line in (Pioneer, Denon or Allen&Heath house mixer for example). Also the RCA cable should not go over too many other cables especially power cables, to minimise influence on the unbalanced cable.
Most “bigger” controllers have either XLR or TRS outputs also. These are mainly used to connect it directly to a PA amp or a PA mixer. Also adapters for converting from XLR to TRS and vice versa are rather cheap and work well, however they are rarely needed as most amps and PA mixers have also both inputs. With these kind of balanced cables you can go over a lot more distance and you run into less problems if you have a lot of cables around, as influences are minimised by design.
2 or 4 decks:
Well most DJs frequently use 2 decks, now many like 4 decks because you can easily prepare more decks and have a switch or “Plan B” set of decks if you need to mixout quickly because the dancefloor is emptied by your current song. Also some like to simply transition 1-2 to 3-4 and back again.
Real mixing with 4 decks is really tiresome, but can be quite fun. I like to do it from time to time, but it is not like I could not live without it and I rarely see DJs doing it for a whole set (actually I cannot remember the time I had seen that in ages). But it is a fun thing to experiment with and you have to decide if you want to miss that or not.
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