Reply to S4 + two X1s + two F1s = ?

Currently, I'm using the DJ Tech 101 Series controllers (sad, I know). But I have downloaded the mappings for them so I can use Traktor Pro 2. So far, Traktor is much better than the software that comes with the 101 Series (Deckadance) just about every single manor. Obviously, controllers designed for Deckadance are very limited when using them for Traktor, so I decided to do much searching for a controllers for Traktor.

I have found that the S4 would be a very good decision for me, however I've also looked at the X1 and F1 for add-ons. I know the X1 is great for controlling traktors effects (obviously) and that the F1 is great for controlling cues, samples, and one shots (obviously. What I'm wondering is:

Should I even buy a X1 if I have an S4?
Are there things that the X1 can do that the S4 can't do? Or are there things that can be done BETTER by the X1?
If so, would there be a use in buying two X1s? I like my layout to be symmetrical to be honest.

Clearly the X1 can do things that the S4 can't do, but again, is there use in buying two of them?

OR would it be better for me to buy an S2 with the combination of two X1s and F1s?

Some people have simply said to buy two of them because "I can do more with them", but what exactly could I do do more with them if I already had an S4?

Any feedback is great, thanks!