Difficulties with 4k60 support on my Late 2013 27" iMac I have a late 2013, 27" iMac with a GeForce GTX 780M graphics card, which I'm trying to get to connect to two 4k displays (Asus MG28U) at 60Hz. When I connect them, I am limited to 30Hz (but importantly, the 3820x2160 resolution does work):



I also have an early 2015 MacBook Pro, which supports these displays at 4k60. This confirms that the cables I'm using (Mini DisplayPort to Display port) are not the limiting factor). Interestingly, the preference pane looks different on my MacBook Pro, offering a bunch of scaling options, with no mention of refresh rate (although I can visually confirm it's 60 Hz):



I am trying to figure out why these two computers treat these monitors differently, and show different settings for them. 

There is conflicting information about 4k support on this Mac. According to Apple's official technical specifications for this (iMac (27-inch, Late 2013) - Technical Specifications), its limited to only 2560x1600:


  Simultaneously supports full native resolution on the built-in display
  and up to a 30-inch display (2560 by 1600 pixels) on an external
  display


This is clearly (no longer) correct, given then my iMac is running two of these displays at 3840 x 2160 at 30hz.

Apple's page on Mac 4k support (Using 4K displays, 5K displays, and Ultra HD TVs with your Mac) contradicts this, claiming that 4K60 is supported for SST displays (emphasis mine):


  With OS X Yosemite v10.10.3 and later, most SST 4K (3840 x 2160)
  displays are supported at 60Hz on these Mac computers:
  
  
  MacBook Pro (Retina, 13-inch, Early 2015) and later
  MacBook Pro (Retina, 15-inch, Mid 2014) and later
  Mac Pro (Late 2013)
  iMac (27-inch, Late 2013) and later
  MacBook Air (Early 2015)
  


...


  Multi-stream transport (MST) displays
  
  These Mac computers support MST displays at 60Hz:
  
  
  MacBook Pro (Retina, 15-inch, Late 2013) and later
  MacBook Pro (Retina, 13-inch, Early 2015) and later
  Mac Pro (Late 2013)
  iMac (Retina 5K, 27-inch, Late 2014) and later
  


Assuming this information is correct (I'm skeptical given the contradictions) this iMac can support 4k60Hz for SST displays, but not MST displays. In that case, I suspect that these Asus MG28U displays use MST, but I cannot find any information regarding whether they're SST or MST.

Has anyone gotten 4k60 to work with a late 2013, 27" iMac? Is there some setting or something that I am missing? Any help would be appreciated.
DisplayPort v1.2 -- vs 1.0 and 1.1 -- newly supported 60Hz UHD (4k), where the older specification had only enough speed to support 30Hz.  [Search "specifications displayport".  (On the Wikipedia page, you have to look at two separate tables.)]

I would infer that DisplayPort port on the older machine was v1.0 or v1.1, with the newer machine's being v1.2 (or higher).  (Apparently your cable is v1.2 (or higher).)

Since the two machines are running the same version of the OS, I would guess that the difference in the presentation of the resolution is a function of the DisplayPort version... although I would have thought that it was the OS itself.  ["Larger text" means (non-native) larger dots on the same screen (and fewer of them), and "More space" means, in the last case, full native resolution.]

As for the Apple specification page... one might wonder if there was an improvement in the hardware capability enabled by the software (i.e. the OS)... but I would guess that it actually was a mistake (surprisingly enough); the NVidia specifications for all the pertinent graphics cards say 3840x2160.  [I note that Apple has been known to cripple its hardware -- as in, such that you can get more than advertised, out of it -- on occasion [ref. incl. my own machine], but I can/do not see that here.]

BTW, my experience with my dual-panel display (which I think is "MST") is that you can catch it out sometimes, with one side lagging behind the other briefly.