The FX850 the way james444 described it (below)
. Otherwise, not sure. Maybe the Dunu DN-1000 if you don't mind moving slightly in a more v-shaped, less smooth direction, or the Sony XBA-H3 for a smoother and more refined (but arguably less clear compared to the Dunu) sound.
Good stuff, FX850 sounds like a winner.
Curious which one that'll be.
Interesting requirements. Not sure I have the solution for you but as a fellow HiFiMan/Etymotic fan I can help you narrow the field.
First off, while I haven't tried all of the models you are considering, I haven't found anything to be a full-on upgrade to the ER4 without compromises (especially in the bass qty vs clarity tradeoff). There are more options in customs than universals that do a good job with this, but still not all of them.
Now to thin the field a bit...
The W40 definitely isn't it - it just doesn't have the same sort of sonic balance. Too much mid-bass and not enough upper mid presence compared to an RE-400 or Ety. The IE800 probably isn't it - it's a rather bassy thing and ends up sounding somewhat v-shaped. It's not really mid-recessed in a bad way, but compared to what you're used to there's a fair chance you'll find it both overwhelmingly bassy and a little unforgiving. The C5 is definitely not it but I can't speak for the Series 2. Maybe it's a 50%+ improvement on the old one.
The SD-2 MIGHT work but its signature is definitely more RE-400 than ER4, meaning it's more on the smooth and warm-ish side than strictly neutral and analytical. It doesn't have any coherence or cohesiveness issues and it's just a nice-sounding earphone with noticeable improvements on the RE-400 here and there, but not a 100% match for what you want. The TDK BA200 is a viable budget/mainstream alternative to the SD-2, with similar caveats.
A couple of others come to mind but all with their own reservations:
The DUNU DN-2000 may be the closest I've heard to a good "analytical" IEM with a sizable bass lift. But again, you do give up some clarity in parts of the spectrum compared to a strictly flat earphone and the very top end is tuned a little brighter.
The Audiofly AF180 seems very promising so far and is a little clearer than even the SD-2, but it has a clarity-adding peak somewhere in the treble just like the DN-2000 which might cause issues coming from an ultra-smooth RE-400. Need to spend more time with this one to form a more conclusive opinion.
ZapX629 provided the link. I do need to re-write all of my innerfidelity reviews and re-post them here; just haven't found time to do it.
