3840 x 1080 scaled not available anymore for M4

[aut0]

I have a really odd problem. I recently upgraded from an MBP with an M2 Pro to an M4. I have an external 5120x1440 49" UW screen.
On the M2 Pro I can get 3840x1080 HiDPI, but on the M4 it selects 3072x864 HiDPI. I tried to add a custom HiDPI resolution in Settings->Display->System Configuration and Flexible Scaling -> Custom scaled resolution, but it does not show up anywhere. Not in "Display Mode" in the BetterDisplay menu nor in the system settings. I tried to reboot, but it still doesn't show up.
Any idea why?

[mojoker]

You ran into the same problem as me, according to apple's official statement, the Mac mini with the m4 chip supports 8k, and my current monitor's native resolution is 51202160, which should logically support 38401440 resolution, but I can only use 3072*1296 hdpi now!

[aut0]

FWIW, I can create a Virtual 32:9 display and then set that to the resolution I want and mirror it to my 49" screen.
That seems to work.
I don't fully understand what I'm doing.
Is there any documentation?

[waydabber]

Currently M4 seems to have 3840px width on 5K2K-s only when the display is 120Hz. This is probably a bug Apple will fix.

https://forums.macrumors.com/threads/5k2k-at-120hz-with-mac-mini-m4.2441289/

[ghost]

Did someone try with macOS 15.4 Beta (2)? Any progress?

[ghost]

I can answer myself:

No, it doesn't help so far. Tried macOS 15.4 Beta 3 on a new MacBook Air M4.

@waydabber is correct: 3840px are working out of the box with a 120 Hz screen, even with the standard M4 (tested with Dell U4025QW).

But 3840px are not available on LG 40WP95X (max. 72 Hz).

@waydabber Are you sure Apple will fix it with an update? AFAIK, the resolutions are chip dependent. I remember your originally findings for M1 / M1 Pro and this was also a reason to create BetterDummy at all.

[waydabber]

No, I am not sure. I think the reason why it's available at 120Hz is that the algorithm that allocates resources may conclude that with 120Hz there is no room left for an additional display to be driven at sufficiently high resolutions anyway so it just concedes giving 3840px to the connected high refresh rate display. With a lower refresh rate display this is not the case so the system reserves additional capabilities for possible additional displays to be connected later on.

The idea behind this is I think the following (I may be wrong):

With M4 there is a new set of horizontal and vertical framebuffer (video memory) size limits (beyond the usual 6K or 8K limit present in previous generations for non-pro and pro chips - previously only a horizontal limit and a max pixel count limit was present based on the hardware limits for a port). The system now dynamically allocates its capabilities when a new display is connected, trying to figure out the max sensible framebuffer size (sadly in a rather conservative/limiting manner) and keep the rest for further displays connected down the road. This allows the M4 generation to support more displays than the previous generations. The shared nature of capabilities is evident from the fact that the number of displays depends on the native resolution of the displays (for previous generations only the port usage was a differentiating factor). All this comes at a cost - those who want higher resolutions (typically Pro users who want large HiDPI desktops on their ultrawides) won't get what they want. Apple could tweak the way this works but it would probably require tweaking the supported mix of displays on the specs page retroactively (and make it convoluted, involving not only resolutions but physical display sizes, taking into account on what displays would it make sense to support higher HiDPI resolutions etc) which might not be something they want to do. But of course in the future they could sidestep the problem and just rethink how desktop composition works in future macOS versions.

[bossdeep]

hi @waydabber is there any update on this? I've been using betterdisplay on my m1pro with no issues, but now upgraded to m4 and 3840x1080 (nor the native 5120x1440) doesnt show me hidpi.

The max i get is 3072 in hidpi

For reference, I'm using a samsung g9 49 inch oddysey.

[waydabber]

Nope. :(

[richlira]

hi @waydabber is there any update on this? I've been using betterdisplay on my m1pro with no issues, but now upgraded to m4 and 3840x1080 (nor the native 5120x1440) doesnt show me hidpi.

The max i get is 3072 in hidpi

For reference, I'm using a samsung g9 49 inch oddysey.

Same, I upgraded from M1 Max to M4 Pro... but now the resolution HiDPI is worse, lol!

[sqqhjqvmnmngud]

I have exactly the same issue, with MacBook Pro M4 Pro and Philips 49M2C8900LA or Dell U4919DW. On M1 Pro I was able to set much more HIDPI resolutions, on M4 Pro the max is 3360x945.