I think that he means with the typical pixel-doubling you get with "retina" branded screens, UI elements on 4096x2160 would actually have the same on-screen size as UI elements on a non-retina 1080p display, which on a 27" screen is probably too big. (27" iMacs are 2560x1600 which makes onscreen elements considerably smaller than if they were 1080p.)
OSX on a "5K" 27" display would maintain the same size of on-screen elements as the current 27" iMac, but everything would be twice as sharp. I would probably buy one in a heartbeat if the price is right.
(Edit: I guess the other alternative is that OSX would use a 4k screen but would do some graphics card tricks to render everything at 5120x3200 and downsample it to 4096x2160, but that's an odd ratio (5/4-ish) so things may not look as good. That's what they do on retina MBP's when you set the display settings to "more space", which renders everything smaller.)
OSX on a "5K" 27" display would maintain the same size of on-screen elements as the current 27" iMac, but everything would be twice as sharp. I would probably buy one in a heartbeat if the price is right.
(Edit: I guess the other alternative is that OSX would use a 4k screen but would do some graphics card tricks to render everything at 5120x3200 and downsample it to 4096x2160, but that's an odd ratio (5/4-ish) so things may not look as good. That's what they do on retina MBP's when you set the display settings to "more space", which renders everything smaller.)