Macrumors has a good overview on the next gen of Thunerbolt Displays and subsequent upgraded (Retina) displays for iMacs. For the 21.5 inch iMac, a true "Retina" (doubling of pixels) would make it exactly 4k resolution (3840 x 2160 pixels) - and thus HiDPI mode would make the entire UI the same relative size, effectively 1920x1080, but incredibly sharp like the current MacBook Pro Retinas. For 27 inch displays, it seems nearly impossible on many fronts to do anything higher than 4k, so they can't just double the pixels like they usually do. DisplayPort 1.2 standard used in Thunderbolt 2 only supports a maximum of Cinema 4K at 4096 x 2160 pixels. This means the 27 inch displays will also likely "just" be 4k. But in order to have a useable UI, the relative UI in HiDPI mode would actually be larger than the current displays. It would essentially be the same situation you see between the iPad Mini and iPad air - same exact screen resolutions, just different sizes. To me, the UI elements and text on current 27 inch displays is actually rather small, so I don't really see it as a terrible thing to sacrifice some screen real estate to run in HiDPI mode. Running at full resolution in 4k would be basically unusable, without a magnifying glass. To get an idea, you can set your resolution on 27 inch to 1920x1080 and see what the relative size of the UI would be - imagine a "Retina" version of that. I do not know if there is a way they could scale the UI to something in between as to not sacrifice as much real estate, and still call it "Retina". My understanding is that OSX doesn't even yet have a setting for HiDPI mode for 4k displays, and that they just run at full scale, making most UI elements unusable. Thoughts?