4K display outputs at 30hz when HiDPI is enabled

The ports on the display are specified as HDMI 1.4 ports. With HiDPI, that is good for nominal 4K, at up to 30 Hz. Those HDMI display input ports are working as specified (essentially unusable).

if your display supported Picture-By-Picture (cousin of Picture-in-Picture, you could connect it with two HDMI cables and place the two half-display back together again. But the specs I found easily did not suggest that display supports that feature.

if you were willing to switch to the DisplayPort or Mini DisplayPort input, DisplayPort 1.2 supports nominal 4K at 60 Hz, and slightly higher (up to around 80Hz).

Cables for DisplayPort at this resolution are limited to ONE meter in length.

Grant Bennet-Alder wrote:

The ports on the display are specified as HDMI 1.4 ports. With HiDPI, that is good for nominal 4K, at up to 30 Hz. Those HDMI display input ports are working as specified (essentially unusable).

I would expect that 30 Hz limit to apply whether you were using Retina / HIDPI "like 1920x1080" or native, non-Retina 3840x2160. In both cases, the Mac would be be drawing on a 4K canvas, and then trying to send video with 4K resolution to a HDMI 1.4 input which might not support 4K at any rate higher than 30 Hz.

https://www.hdmi.org/spec/hdmi1_4b

Selecting "1920x1080 (low resolution)" would allow a higher refresh rate, because the signal going to the HDMI input would only have 1920x1080 pixels. It would be the monitor itself blowing the signal up to fill the screen.

Even HDMI 2.0 can impose limits on 4K displays. If you have a 4K display that is capable of 10-bit-per-channel color, HDMI 2.0 will not let you have all of

• 4K resolution
• 10-bit-per-channel color
• 60 Hz refresh rate
• RGB 4:4:4 encoding (where each pixel gets to have its own color)

at the same time. You might have to give up 10-bit-per-channel color to get a 60 Hz refresh rate, or vice versa.

I was having a serious issue trying to get the MacBook Air M1 to display on a 4K Lenovo screen. I fixed it by selecting the 1280 x 800 resolution on the Mac and setting the Mac as an extended display, then when I clicked the tab for the 4K Lenovo and clicked show all resolutions, and a list of all resolutions came up at 60Hz. If the settings are not exactly like this you will get a different list of resolutions including HDPI and those are at 30Hz only. When you set it up like this you can then close the MacBook and the setting for the MacBook screen will disappear. All the resolutions available that are not labeled "low resolution" show as crystal clear regardless of how small the details are because it is a 4K screen.

not recommended, unless there are no other alternatives.

Most users are choosing a USB-C/ThunderBolt to DisplayPort adapter/cable where BOTH the USB portion leading into the adapter electronics AND the DisplayPort portion from adapter electronics to display, are each under one meter.

Native resolution 

3840 x 2160 

Input connector 

1 HDMI 1.4;

1 MHL 2.0 /HDMI 1.4;

1 DisplayPort 1.2;

1 Mini-DisplayPort

what cables, adapters and docks are you using to make the connection? please mention every device.