I would've commented, Hans, but I don't have a 4K Retina display. I kept ignoring the topic going by the title.
I'm stuck with a 1080 LG 27" hooked to a 2010 Mac Mini. I don't have your problem and frankly your outline of your issue was quite difficult to keep straight in my head due to your detailed account of what 4K scaling does to 1:1 previews in Lightroom.
I can only think about how tiny each RGB pixel site looks on a 4K screen and trying to visualize how Lightroom is going to draw all those high rez DSLR pixels contained in your images to screen dots that can't even be seen because they're too small. The antialiasing preview algorithms for scaling up or down has got to be a nightmare.
Even on my 6MP DSLR images at 1:1 LR previews don't give me an idea how sharp it's going to look when I downsize it for web display. I zoom down to a level that invokes antialiasing that gives the closest match to downsampling to 700 pixels on the long end to give me an idea how sharp to make the image.
I also have output sharpening set to Glossy Paper/High whose affect doesn't show up in 1:1 previews. See? Look at all the variables I just came up with. It's confusing as hell.
A 4K screen has 4x more pixels than yours and the Dell 32" monitor is a bit larger, but it is probably not worthy of being called retina since the PPI is much less than the 15" MacBook Pro retina screen or the 27" 5K display on the new iMac. The PPI of the 32" 4K display is 138. The PPI of the MacBook Pro 15" is 220. In comparison a "standard" 30" 2560x1600 display is 100 PPI.
The reason I wrote this up was to make others aware of the issue of scaling in OS X relative to 1:1 display in Lightroom which is to judge pixel level sharpness in your photos. I felt it would be useful to make others aware of this, if they had not already seen this issue. I could also be read as a warning against getting a 4K display.
I don't have an iMac 5K but I would expect that with the resolution set to default which is 2560x1400 that 1:1 would map pixels from the image to the screen pixels 1:1, but other scaled resolutions would not and therefore make the 1:1 the wrong size on the screen.
You other sharpening considerations has in my opinion not really anything to do with the issue I brought up