For LG OLED users: Mac mini M4 does not support HiDPI 4k over HDMI 2.1

[dinosauradventure]

EDIT: 4k @120HZ is working with M4/M4 Pro. This thread is about running at a scaled/downsampled "looks like" 3840x2160 resolution, which is not currently supported on M4.

I am starting a new thread with all known information about the lack of 4k HiDPI support with the M4/M4 Pro Mac mini, as there is a lot of varying information floating around. I wish I knew about this limitation before purchasing my M4 Pro mini, so want to make sure others are aware of the issue. It has an HDMI 2.1 port but has some annoying limitations.

What is HiDPI? HiDPI, or High Dots Per Inch, refers to a display mode that renders in 2x or greater resolution, scaled to fit the display’s native resolution. HiDPI is also known as Retina, a marketing term used by Apple. More info at this link:

A Definitive Guide to External Displays on macOS​

Summary of issue:

• M4 and M4 Pro Mac mini do not support HiDPI 4k/5k over HDMI. BetterDisplay (and similar apps) cannot fix the issue; these resolutions appear to be software blocked by Apple. Tested in all versions of Sequoia (including latest Public Beta 2).
• This is despite 8k/60hz fully working over HDMI (which is the same horizontal resolution as HiDPI 4k).
• The flashed Cable Matters VM7100 method described in this thread for 4k/120hz also does not work on M4 minis. It has the same HiDPI limitations as built in HDMI. This method is still working on M1 Macs in Sequoia if you set 60Hz in system preferences before using BetterDisplay to enable the unlisted refresh rate of 120hz.
• If you plan to use an LG Cx OLED with your new mini, prepare yourself for some very big UI scaling as it only has HDMI inputs and cannot run at native 4k resolution with HiDPI. Other scaled resolutions (3360x1890 and below in my experience) work fine in HiDPI with 120hz.
• If you want to run a 5k2k display over DisplayPort, it's reported working with a specific cable (see this post).
  I can't confirm for sure but it seems like DisplayPort connections are not affected by this issue. I assume that any 4k monitor connected with DisplayPort should have HiDPI.

Please consider submitting feedback to Apple about this issue at feedbackassistant.apple.com.
You can reference ticket #FB15750642 that I already opened regarding this issue.

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Here are the resolution details for my machines connected to a 42" LG C2:
MBP M1 Pro connected with flashed USB-C adapter:
View attachment 2451377
An here's the M4 Pro mini connected to built in HDMI 2.1:
View attachment 2451372

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Technical details below. Thanks @waydabber for the investigation and information. Please correct anything I got wrong.

8K@60Hz and 4K@120Hz and 144Hz worked fine using the built-in HDMI port. However HiDPI resolutions are strangely fluctutaing/limited based on the display configuration (running 15.1) indeed, (both using HDMI and USB-C). The reported GPU horizontal limit is 3840px HiDPI (which is an improvement for entry level Macs) but similar to how refresh rates are limited in the display mode list for some models, resolutions seem to be limited as well. This is not entirely new as the same is the case with my older M2 Max Studio over HDMI with some displays interestingly (where the normal mode list contains no options beyond 2560px HiDPI on a 4K HDMI display while the machine should not have a limitation like that). So there is an extra logic that seems to limit HiDPI scaled resolutions below what the system is clearly capable to do.

(note: HDR resolutions are also limited with high refresh rate in the same manner as on older systems).

The M4 can also output 4K@120Hz (can do it up to 240Hz actually) just fine, but HiDPI resolution still seems to be limited at 3072px horizontally (for all refresh rates) for 4K displays for some reason. Obviously this is not the case when I connect a 8K display over HDMI (like a Samsung 8K TV) - in this case the mini has no issues going up to 3840px HiDPI horizontally, since this is needed for a proper 8K output. Also, the horizontal GPU limit according to the ioreg framebuffer entries is 3840px horizontally which is an obvious increase from the previous 3072px for entry level macs, so the limit in display mode list is clearly artificial...

If I add a 8K@30Hz timing, the HiDPI resolutions all the way up to 3840px horizontally appear - however the scaled resolution base (native resolution) will also be 8K which a 4K obviously can't handle. Sadly if I override the native resolution to be 3840x2160 again, the higher-than-3072px modes simply disappear from the mode list (even though the 8K mode is still available in the low level connection mode list and can be triggered, so the Mac parsed it from the EDID all right).

So all in all this seems to be a software limitation. I see no way around it now.

 

[Cosmo M3]

Noticed this on Day 1 of my Mac mini ownership as well. Thought I was going crazy why HiDPI wasn’t working at native 4K on my 42” LG C2. I’ve learned to live with it as it still plays HDR content where it matters.

 

[aiyaaabatt]

Wow, this might be a deal breaker for me. Do the Mac Studios have the same issue?

 

[chrfr]

Wow, this might be a deal breaker for me. Do the Mac Studios have the same issue?

The Mac Studios are still M2 based.

 

[waloshin]

The Mac Studios are still M2 based.

Probably had nothing to do with M2 or M4 probably Apple making another market segmentation… you want HiDpi then you buy a “pro” studio.

 

[blufrog]

What is HiDPI and how does it relate to 4K?

I have a 4K monitor (3840x2160 resolution). It only supports 60 Hz over HDMI. What does HiDPI do?

I was considering buying a 4K 120 Hz TV (LG G4) but it seems if I do, I'm going to be hitting some scaling limitation???

If I read correctly, DisplayPort is unaffected? Why would that be?

 

[blufrog]

Duh... HiDPI = scaling.

Well that sucks. So it isn't possible to set the desktop to native resolution and use scaling?

 

[dinosauradventure]

Duh... HiDPI = scaling.

Well that sucks. So it isn't possible to set the desktop to native resolution and use scaling?

Thanks, I’ll add a definition to the OP!

That’s right, you can run native 4K but only at 1x scale, which looks very pixelated. Text looks especially bad due to the lack of subpixel antialiasing in macOS. I will update the OP later with some pics of how it looks, compared to my M1 MBP at 2x scale.

I can’t confirm that DisplayPort fully works as I don’t have a monitor to test with, but reports seem to indicate better support.

As to why this limitation, we don’t know, it may be for performance reasons. Waydabber indicated this may be to reserve bandwidth to support 3 displays as they advertise. But considering that they allow full 8k, it doesn’t make sense to me. A few of us assumed that Apple does this to push people to buy Apple displays, however the Pro Display XDR is reported to have similar limitations (even with thunderbolt connection).

The smaller scaled resolutions look sharp, and some people even prefer them. But for me it’s just not enough space, I like to have 4 windows on screen at a time and need as much space as possible.

 

[blufrog]

Hmm. I'm going to need to re-evaluate buying "just any display" then.

The comment about the Pro Display is interesting - I looked at one the other day in the Apple store connected to a Mini, and I thought it looked ... sharp. I wonder if this is why? Best way I can describe it as everything lacking antialiasing, as you describe. It didn't seem to be running at 120 Hz, either - motion didn't seem fluid enough, but I didn't get the chance to dig into the settings.

I think a research trip is in order.

As old as it is, I'm starting to feel really spoiled by my iMac 5K. The display is one of the things I love about it.

 

[jowow]

I am using MBP M2Pro with LG C2 at 1920x1200 display scale and planning to buy Mini M4pro. So it is fine for 1920x1200 scale with new Mini ? Thanks !

 

[blufrog]

I am using MBP M2Pro with LG C2 at 1920x1200 display scale and planning to buy Mini M4pro. So it is fine for 1920x1200 scale with new Mini ? Thanks !

What is the native resolution of that display???

 

[Cosmo M3]

What is the native resolution of that display???

Dude the native resolution is 4K.

 

[dinosauradventure]

Hmm. I'm going to need to re-evaluate buying "just any display" then.

The comment about the Pro Display is interesting - I looked at one the other day in the Apple store connected to a Mini, and I thought it looked ... sharp. I wonder if this is why? Best way I can describe it as everything lacking antialiasing, as you describe. It didn't seem to be running at 120 Hz, either - motion didn't seem fluid enough, but I didn't get the chance to dig into the settings.

I think a research trip is in order.

As old as it is, I'm starting to feel really spoiled by my iMac 5K. The display is one of the things I love about it.

I feel you. The Pro Display XDR is only 60hz which is crazy for the price.

I also loved the iMac 5k display. I just bought a used studio display to try out... If I like it I’ll buy a 2nd. They are also 60hz but I might just live with that at this point, I don’t do gaming on Mac just web design work. The price hurts for what is basically a 10 year old iMac display.

I’d really prefer one big (32”+) 6k screen, with OLED and 120hz. But it doesn’t exist. I don’t have confidence that Apple will release new displays with any of this tech at a reasonable price anytime soon.

I am using MBP M2Pro with LG C2 at 1920x1200 display scale and planning to buy Mini M4pro. So it is fine for 1920x1200 scale with new Mini ? Thanks !

I think you mean 1920x1080 right? If so, yes it works with HiDPI and looks good.
LG C2 is 16:9 and 4K native (3840x2160).

 

[blufrog]

Dude the native resolution is 4K.

This is why I asked:

>> I am using MBP M2Pro with LG C2 at 1920x1200 display scale

...because I thought it was a 4K display. The part that threw me was he said "1920x1200". I missed the SCALE part.

In that case, it seems the answer is currently NO, it won't work as expected.

 

[throAU]

I feel you. The Pro Display XDR is only 60hz which is crazy for the price.

It's not for a display with the colour accuracy.

 

[Significant1]

Give SwitchResX a try.

SwitchResX - The Most Versatile Tool For Controlling Screen Resolutions On Your Mac

And check possible Apple Silicon limitations in the faq

SwitchResX - FAQ

 

[blufrog]

I feel you. The Pro Display XDR is only 60hz which is crazy for the price.

I also loved the iMac 5k display. I just bought a used studio display to try out... If I like it I’ll buy a 2nd. They are also 60hz but I might just live with that at this point, I don’t do gaming on Mac just web design work. The price hurts for what is basically a 10 year old iMac display.

I’d really prefer one big (32”+) 6k screen, with OLED and 120hz. But it doesn’t exist. I don’t have confidence that Apple will release new displays with any of this tech at a reasonable price anytime soon.


I think you mean 1920x1080 right? If so, yes it works with HiDPI and looks good.
LG C2 is 16:9 and 4K native (3840x2160).

In that case then it just saved me £1500 in not needing to buy a new display of any kind. My current 4K display is more than good enough.

I was tempted to get a 120 Hz display for the Mini and my PC, but it seems it won't allow scaling at 4K native resolution, meaning the display will need to run at a non-optimum resolution, which doesn't look great (or be tiny and jaggy).

 

[blufrog]

It's not for a display with the colour accuracy.

As well as the color accuracy, I like how smooth iMac displays looked.

My iMac 5K sat next to my 4K display makes the 4K display look like you used sandpaper on it (and I don't mean the screen finish).

 

[Cosmo M3]

I was tempted to get a 120 Hz display for the Mini and my PC, but it seems it won't allow scaling at 4K native resolution, meaning the display will need to run at a non-optimum resolution, which doesn't look great (or be tiny and jaggy).

It does, you just won't get HiDPI.

 

[bzgnyc2]

I can’t confirm that DisplayPort fully works as I don’t have a monitor to test with, but reports seem to indicate better support.

Has anyone tried connecting through a USB-C/DisplayPort to HDMI adapter? Seems an unfortunate waste of a TB4/5 port on a system already 1 down one from the Mac Mini 2018 and Mac Mini M2 Pro but might be a viable workaround for many.

As to why this limitation, we don’t know, it may be for performance reasons. Waydabber indicated this may be to reserve bandwidth to support 3 displays as they advertise. But considering that they allow full 8k, it doesn’t make sense to me. A few of us assumed that Apple does this to push people to buy Apple displays, however the Pro Display XDR is reported to have similar limitations (even with thunderbolt connection).

It's especially unclear to me why they would throttle options when using HDMI as an output relative to DisplayPort over USB-C for output resolutions clearly within the HDMI 2.1.

 

[dinosauradventure]

Has anyone tried connecting through a USB-C/DisplayPort to HDMI adapter? Seems an unfortunate waste of a TB4/5 port on a system already 1 down one from the Mac Mini 2018 and Mac Mini M2 Pro but might be a viable workaround for many.

I have tested both a USB-C to HDMI adapter and a USB-C to HDMI 2.1 cable. Same result, no 4k HiDPI.

 

[blufrog]

If it is because of supporting triple monitors that they disabled scaling at 4K or higher, then surely they could be smart enough to see how many monitors are plugged in, and allow it if only one or two are in use?

I can understand them wanting to disable it if performance becomes an issue, but it seems a bit of an odd thing to disable outright.

 

[blufrog]

Another question: does this happen with MBP M4 Pro? Max?

 

[bzgnyc2]

I have tested both a USB-C to HDMI adapter and a USB-C to HDMI 2.1 cable. Same result, no 4k HiDPI.

Such a weird limitations -- it's like Apple hates HDMI now.

One other possibility -- perhaps your USB-C -> HDMI adapter used HDMI alt mode rather than DP alt mode? In which case, what about an USB-C to DP adapter connected to a DP to HDMI adapter?

Also since this is a random limitation, one random thing to try: can you try forcing the display output to RGB instead of YCbCr?

 

[Cosmo M3]

Another question: does this happen with MBP M4 Pro? Max?

this is a Apple thing, regardless of device.