Only one way to really find out, get both and listen. This whole thing about bass impact vs. depth is all very interesting, but I have seen a few, not many, HFers who didn't find the RE0 to be as magical as others, and have sold them or returned them.
No one, at least not me, is saying the S4s are replacements for the UM3X (in my case), but I do prefer the S4 over the X10, another Klipsch phone, which had an MSRP of $349 out of the gate. I think personal preference for the RE0 is perfectly understandable, and there are some other serious RE0 fans here (mark2410, pianist to name two). But there are also a few folks who have had top tier IEMs (I have UM3X and had W3 and IE7 and IE8) who probably would not be so smitten with the RE0. According to the man who owns Head-Direct, the new RE252s are superior to the RE0. I listened to my new pair of RE252s today, before they sort of broke (getting a replacememt, some bad glue work), and they too sounded like they had some amazing detail, but again, without EQ or an amp, the bass just was not there, and I am not talking heavy, thumping bass. More like Ety ER4P bass.
Again, I've owned Etys and right now the 616s in my ears with the Sony X that showed up today, and the detail is amazing since they have Ety drivers. But I had to boost clear bass to make them really hit their stride.
Why are the RE0, now a $79 IEM, allowed to be "giant killers" because of their detail, but other phones that have different sound signatures don't qualify? Is detail the end all and be all of what makes a great IEM? Not necessarily.
I know the RE0 was originally $199, but their value has not held - unlike true top tier phones.
In the end, it's always personal preference, but to dismiss the S4 as clearly inferior to the RE0 as a listening experience as if there is some quantitative measure is not really meaningful. I feel there is room in my listening world for the UM3X, the S4 and the $14 im616. Probably the RE0 too. But since the 252 is being touted as superior by the man who owns the company, I guess I will have to use the 252 as my guide.
In the end, I would not be surprised if the OP gets the RE0 and immediately says, "Where's the bass?"