The UERM is a phenomenal IEM; the ER-4S is also a phenomenal phone, one I regard as a sonic masterpiece (a masterpiece for the simple reason that it's a single-BA phone, and one that's been around for 22 years and is still a true top performer).
Both UERM & ER4S have their advantages & disadvantages. With the former, going down the customs route can prove a very frustrating affair (possible refits, getting good ear-impressions, poor re-sale value, occlusion effect (also known as bone conduction), extra expenses, particularly if one doesn't live in the USA, long-ish wait times, not to mention a price-tag of $1,000 for the IEM itself…then, of course, add the fact you can't let others enjoy your UERM as they're custom-made for your ears.).
The ER-4S retails for $300 (but can be had for less as is the case with most universal IEMs—though, in fact, when the ER4S was released it cost an arm and a leg!). Getting a good fit & seal, however, can prove more than a challenge — this IEM requires a deep, deep, DEEEP fit to get the target SQ/FR, and that's not something everyone can handle (comfort-wise), and even then, a good seal is not always guaranteed (!). Add to that a pretty microphonic cable—a shirt-clip is a must—and the ER4S isn't a phone you can easily recommend. So, the ER4S, as great sounding as it is, can also be a royal pain in the a**e.
Enter the F111. Apparently the F111's FR was inspired by the legendary ER4S. Yet, on closer examination, they're sonically close, yes, but not that close — low-end performance is better on the F111**, the midrange is more linear than on the ER4S, but the treble is a little more polite and does not extend as high up as the ER4S, but, overall, treble on the F111s is really very good.
These days the F111 can be had for ~$500 (~$430 if you live in Japan). FitEar is known for their excellent workmanship (read: impeccable build) — the F111, like all custom IEMs, is actually hand-made, so even though it's a universal IEM, it's not mass-produced; it comes with removable cables—FitEar connectors are probably the best in the industry—, and though the cable itself on the F111 is not as premium as those found on the more expensive FE universals (Parterre & TG!334) or FE customs, I personally much prefer the F111 cable as it's quite a bit more flexible, non-microphonic, tangle-free and pretty easy to store (the Parterre & 334 cables are very nice but, in the end, a bit of a pain to use in the real world). But, perhaps the best news is that, unlike with the ER4S, with the F111s one can get a consistently good fit and optimal SQ very easily, no need to rape your ears (read: the F111s do not requite such a deeeeep fit).
**I was one of the earliest F111 adopters—when this IEM was actually called "TO GO! 111"—and though I liked it quite a bit then, I sometimes felt it was a little lean-sounding, lacking low-end presence. At the time, I found myself reaching for my FIBASS much more often. 14 months later, I decided to buy the F111s again (have had my current pair for 8-9 months), and this one does sound better, one I definitely prefer over my FIBASS — my guess is FitEar altered the tuning of this very fine phone.
Second thoughts about a phenomenal custom (UERM)? Second thoughts about a universal legend (ER4S)? I say get the best of both worlds: the EXCELLENT F111. (I've owned the much more expensive TG334 & Parterre, and the F111, FWIW, SQ-wise, is my favourite—the only FE IEM, in fact, I've decided to keep.)
