I have the ER4XR, and previously owned F4100.
The best comparison I can give is this: the ER4XR has better technical abilities across the frequency spectrum, and is more detailed. However, the F4100 has some kind of addictive musical quality that I can't quite put in terms of technical ability. Something about it was very attractive for songs with a lot of midrange energy, but that would be it. Overall I would recommend the ER4XR for its versatility and technical ability. The F4100 would be a good curiousity purchase, but it is lacking in detail and versatility (highest highs and lowest lows are not the greatest).
In terms of durability, the ER4XRs win by far. I have had mine for >2 years, and aside from cable replacements (due to biweekly doorknob cable snags), they work flawlessly (I even wear them in the shower, not under running water but humid environment regularly). The F4100s were good for about a year, and then the right side gave out.
The ER4XRs are quite comfortable too, subtle cable and lightweight IEMs. The Isolation is also top tier (you would be extremely hard pressed to find IEMs with TOTL sound AND over 35 dB isolation). An IEM with Etymotic form factor and, say, Andromeda soung sig would be endgame
