Listening to the Flat-4 now. First tentative impressions: treble spike. It actually reminds me a lot of the JVC FXD80, only with even more emphasis at~5k and ~8k. It's pretty difficult to avoid the harshness. I need to listen at a much lower volume than I typically would with the ER4S. I settled on medium DBA-02 tips which stretch nicely over the barrel of the Flat-4 and seem to take a little of the edge off for me. Mids are what sound most similar to the FXD80 from memory, present and detailed, but definitely behind the treble and bass. The shallow fit also reminds me of that JVC line.
The Flat-4's cable is absolutely amazing. It's like the CK10 cable, with a lot of pliability, ample thickness and weight, but a little more slick, which makes it even less tangle prone imo.
Now first impressions of the IE800: MUCH better. Definitely hear the dip in the mids, and they're a bit warmer because of having less 2k-5k emphasis than something like the ER4S or F111. Somehow it seems to work on the IE800 though, and vocals don't sound recessed, even on tracks known to have less vocal emphasis. Surprising! Even boosting 3k didn't sound quite right for some reason. I don't think it works with the treble peaks.
The biggest treble peak isn't until above 10k to my ears, which allows the IE800 to avoid harshness imo. It's a little peaky overall, but definitely nowhere near the Flat-4 to my ears. Cymbals can have a tendency to sound brittle, with the right shimmer but not enough body.
Compared the ER4S, the IE800 is massively different. The ety is much more "open" and much more detailed. The IE800 moves more slowly, with a bigger soundstage, and just a totally different goal with it's sound.
More to come in a few days.