You said you had an ACD. The TBD resolution is 2560X1440, which isn't far removed from 4K. Is the 27" ACD the same res? l've only had a 24", so my experience isn't comparable.
The 27" "LED Cinema Display" is basically the Thunderbolt Display without the Thunderbolt and is the same 2560x1440.
UHD (what most manufacturers are now selling as 4k)is 3840x2160 - 50% more pixels-per-inch than 2560x1440 - I wouldn't call that "not far removed".
At home, I've just replaced a 24" 1920x1200 with a cheap-ish Dell S2817Q 28" to "try out" 4k on a Hackintosh. - I expect to get either a 5k iMac or some other new Mac + a higher-end 4k or 5k display as soon as Tim unblocks his pipeline and use the Dell as a second screen.
...and, yeah, it blows the 24" away on sharpness and screen real estate.
Mostly I use it in scaled "Looks like 2560x1440" or "Looks like 3008x1692" mode - the "looks like" is misleading because although the system fonts, icons etc. come out the same size as 1440p they're made out of 50% more pixels and far sharper (as long as you're using retina-aware software, which is most things these days) - and of course most software that you're likely to use lets you zoom the content independently, and will take advantage of the extra pixels.
With the 28-inch screen "native" 3840x2160 is actually bordering on usable - I can certainly read it a but my eyes probably wouldn't thank me after sustained use.
So, if you're looking for maximum screen estate for coding etc. you might want to think about going for 30 inches or so. Or having the eyeballs of a 21-year-old.
At work, I use an ACD and, no its not as sharp as 4k, and lacks the slightly finer "scaled" modes that could be useful for extra real estate but... its still a ruddy good display, so upgrading it wouldn't be my first priority.
Other thing to consider - if you've got older "nonretina" Macs that you want to use with the display, that output a native 2560x1440, then although the display
will upscale them, the result is very "soft" looking and not a patch on what you'd get with an ACD, let alone the "scaled" modes on a Retina-capable mac. My 2011 MBP can actually manage 4k, but only at 30Hz which makes cursors and dragging somewhat jumpy.
NB: Dell S2817Q quick summary:
pro: 28"
, relatively cheap; sharp, clear 4k picture, DP/miniDP & 2xHDMI ports, internal speakers good by internal speaker standards.
con: washed-out colours, no adjustable stand (which also makes connections a pain), no VESA mount, fiddly/poor quality menu buttons - summary: great for general use and coding, probably not for artists. I'll be happy delegating it to "second screen" when I upgrade my Mac. 2011 MBP wouldn't drive it via DisplayPort, but an active MiniDP-to-HDMI adapter worked.
