The Sony 4Ks are only 30Hz at 3840x2160 as well, because they only have HDMI 1.4 inputs. HDMI 2.0 is not likely to be out until later this year or into next. Then you have to wait until there are TVs that will handle it. LG has promised that their 4K TVs will be hardware upgradable to HDMI 2.0, but they are even more expensive than the Sony’s.
(Unfortunately, the Sonys have their fancy speakers at the sides, extending the width considerably. They might sound great, but they might not be comparable – in performance or familiarity – to the nearfield monitors you have now.)
We watch BluRays on our TVs at 24Hz, so Cubase’s fairly static display should not be a problem. If you do video, I suggest putting its window on its own display. No point in paying to make your video handle your whole 4K display when only a small part of it requires the performance - subcontract out your video to a cheap small TV, or if the 4K you get can handle picture-in-picture (Seikis can’t, and on some TVs, the antenna has to be one of the sources), feed one of the video card’s outputs with the video window into it as a ‘separate’ monitor.
As for font scaling and comparisons to ‘retina’ displays, if the pixels on the 4K monitor are roughly the same size as on the current monitors, there is NO ISSUE with scaling, because there is no need to scale. A 40" 4K TV is like 4 x 20" 1920x1080 monitors, and a 50" is like 4 x 25". The pixels sizes would only be a few % different in size to most monitors, and hence neither of these would require scaling for use with computers.
The issue with retina displays is that the pixels are much smaller than on normal monitors, because they are meant to be too small to be seen individually, but this conflicts with OSs like Windows and OSX, that have relied upon the pixels being sharply seen so that text can be readable. Instead, all dialogs and icons end up being tiny, and hence the requirement for everything to be scaled up so that they look like the size we are used to.
We are keenly interested in going 4K, but ideally we would want to be able to use them as TVs as well, reducing monitors and a TV to one. While the Seikis have been praised for the look of their 4K screens, they are not so keenly received as to their standard Full HD performance. However, their price is low enough (especially compared to my Dell 30"s) that they could be used as monitors for a year while the market sorts itself out. 4K is twice the pixels that each Dell 30" (2560x1600) has, but one large monitor is way more flexible.
Now we just have to wait until they (or reasonable ones from another Chinese OEM) are available in Australia.