Few days ago I saw in the AnandTech test that the 2 560 x 1 440 mode was not possible in HiDPI with a 4K screen. Nevertheless, with the help of the developer from SwitchResX, an excellent utility , I succeeded.
A 4K Ultra HD screen. I used a HDMI 1.4 TV, it should also work with a DisplayPort display.
A Mac with a Kepler-type nVidia graphic card (most of the GeForce 600 and the GeForce 700). In my case, a GeForce GTX 780 connected in Thunderbolt.
SwitchResX in its next version. I used a beta which is not public yet.
Be careful : it does not work with Intel GPU and AMD GPU, which explains why Anand failed to work in this mode.
Then it’s simple : create a scaled wide definition (then, larger than the resolution of the screen). In our case, 5120 x 2880. Then, reboot and – if all goes well – Mac OS X accepts this definition.
Finally, you can switch to 2560 x 1440 in HiDPI (Retina).
The interest is simple : we keep a good work surface while having a very defined image. If we can notice the resizing when approaching my test screen (a 65 inches in diagonal TV), it must be invisible on a smaller screen, such as 24, 28 or 32 inches models.
In practice , Mac OS X will double the size of the interface elements and make a report in 5120 x 2880 in his memory, with a work surface equivalent to a 2560 x 1440 screen. Then, this 5120 pixels wide image will be reduced to 3840 pixels wide before being displayed. This is the same technique used on the Retina MacBook Pro to propose 1920 x 1200 Retina on a 2880 x 1800 display.
The result is a very clean and neat image, and a good work surface.
We can assume that Apple will offer this feature with a future 4K Ultra HD screen, the successor to the current Thunderbolt screen.