Well the 4.Ai won't be doing any better vocal-wise, not to mention the stage is a bit less coherent, overall a bit more warmed-over. They'd probably be even less forward than the UEs. And the W4s, whilst being extremely proficient vocal-wise, are pretty much the anti-thesis to tight bass (it's nice but relative to the UEs...) and airiness. This is all without added impedance of course, otherwise the signature would likely get around to what you're looking for, but the power-siphoning and general inconvenience would be a rather significant deterrent for portable use.
At this point, you're pretty much at the end of your ropes. Personally I might go back to the UEs and experiment with EQing if you haven't already, since they had pretty much satisfied everything else you were looking for. Otherwise, the only other things left would be the Stax IEMs, which are a real hassle for RL usage, although the sound should be airy enough (don't quote me on this, I've only heard a friends briefly), or perhaps the F111 (if rumors about them being similar to the ER4P has substance) but can't give any direct experience on that front as, well, my DAP battery died as I was trying it
Some more unconventional ways to go would be to just bite the bullet and aim for customs, as the amount of time and money you're using experimenting on UIEMs could possibly be completely remedied by spending good money on something like the UM Miracles (demos have a lot of the attributes you're enjoying on the UEs from memory, not to mention significantly more upper-mids/female vocal registers).
And of course, if the RE-400s I have here are any indication of how the RE-600s will turn out, might just wait for that. Something else you might be interested in as well could be the Tralucents. I might have access to a pair later on to demo, and the general impressions seem to lean towards naturalness, large immersive stage, a slight U-shape. Will let you know how that goes.