4k displays (ie. 3840x2160 resolution) are all the rage nowadays. More and more display manufacturers are making their own 4k products.
There is one thing that I have noticed about many of them, however: Many, even most, of these displays are using HDMI 1.4, rather than HDMI 2.0. Which makes little sense.
The major difference between the two versions is bandwidth. HDMI 1.4 does not have enough bandwidth to display 4k video at 60 Hz (in uncompressed RGB format). It only has enough bandwidth to do so at 30 Hz. HDMI 2.0, on the other hand, has the required bandwidth for 4k@60Hz.
It's never a question of the display itself being incapable of displaying 4k content at 60 Hz, as invariably they support this through their DisplayPort connection. It's only the HDMI connection that limits the refresh rate to 30 Hz.
Some 4k displays do support HDMI 2.0, but for some reason they seem to be a minority at this moment.
This is problematic for several reasons. Firstly, it forces PC users to use DisplayPort rather than HDMI. Ok, perhaps not such a big deal.
But secondly, and more importantly, both the PS4 Pro and the Xbox One X have only an HDMI 2.0 output port. They do not have DisplayPort support. This means that you can't use one of these HDMI 1.4 displays with them, if you want 60 Hz in RGB mode. (At least the PS4 Pro supports 4k@60Hz with HDMI 1.4, but only in YUV420 mode, which has reduced lossy colors, making colors less vibrant and with artifacts.)
I can't really understand why monitor manufacturers are doing this. Sure, they probably have to pay more in order to use HDMI 2.0 (AFAIK it's not free to use), but I doubt it's that much more.
Moreover, many manufacturers are outright hiding which version of HDMI their monitor is using. Many of them only list "HDMI" as supported input, without specifying the version number, anywhere. If you want to be sure and find out, your only recourse is to try to find some third-party review that mentions this.
Although, at this point, it's probably safe to assume that if the monitor manufacturer is not telling which version of HDMI they are using, it's probably 1.4.