Question about M4 and 5k2k displays

[lewismoss]

I've seen this MacRumors thread that is commenly referenced here about the M4 and 5k2k: https://forums.macrumors.com/threads/5k2k-at-120hz-with-mac-mini-m4.2441289/

I used ChatGPT in agent mode to go through and summarise the whole thing. At the bottom is what it said.

I am going to get an M4 Macbook Air and already have a Samsung G75F 5k2k. It will be interesting to see if it natively works. If not, I was wondering if the below 'EDID hack' is actually a real thing that would work? Can I somehow obtain a Dell U4025QW EDID and force that for my Samsung G75F, and then get native 3 840×1 620 HiDPI without mirroring?

ChatGPT:

After reading the entire MacRumors thread (all 29 pages) I found that most participants initially assumed the M4 Mac mini would handle 5K2K (5 120 × 2 160) ultrawide monitors at 120 Hz easily because Apple advertises 8 K60 and 4 K240 support.  Bandwidth calculations by users like joevt show that 5 K2K at 120 Hz uses less pixel clock (~1.4 GHz) than 8 K60 or 4 K240, so the display controller in the M4 clearly has enough headroom .  Early tests confirmed that the M4 Mini could output native 5 120 × 2 160 at 120 Hz when connected via a high‑quality DisplayPort 1.4/Thunderbolt 4 cable that supports Display‑Stream‑Compression (DSC) .  However, the discussion uncovered several important limitations:
• Scaled HiDPI resolution is software‑limited.  Apple uses a new MaxSrcRectWidthForPipe property in the framebuffer driver.  On the base M4 mini this exposes two “pipe” limits: 6 144 px and 7 680 px horizontally.  Many ultrawides trigger only the 6 144‑pixel pipe, so macOS will not offer scaled HiDPI resolutions wider than 3 072 px (i.e., 3 072×1 296) even though the hardware could handle 3 840×1 620 .  waydabber (the BetterDisplay developer) measured that 3 840×1 620 HiDPI is available on the M4 but macOS will only enable it if the monitor’s EDID advertises 8 K/120 Hz modes—otherwise the OS sticks to 3 072×1 296 .
• 3 840 × 1 620 HiDPI is tied to 120 Hz refresh.  From the mode lists, 3 840×1 620 HiDPI appears only in 120 Hz and VRR (48–120 Hz) modes; there is no 60‑Hz or 75‑Hz variant .  Several LG and older Dell monitors can deliver 5 K2K only at 60/75 Hz; because macOS refuses to offer 3 840×1 620 at 60 Hz, those monitors never show that HiDPI option .  Users can select native 5 120 × 2 160 or a 3 072 × 1 296 HiDPI mode, but cannot get the “sweet‑spot” 3 840×1 620 HiDPI scaling.
• Monitor/EDID‑dependency (“whitelisting”).  The Dell U4025QW is currently the only 5 K2K ultrawide that works out of the box.  It advertises 120‑Hz 5 K2K modes and 8 K DSC in its EDID, so macOS uses the 7 680‑pixel pipe and offers 3 840×1 620 HiDPI at 120 Hz .  Other monitors (Dell U4021QW/U4024, LG 40WP95C/34BK95U, Kogan and Kuycon 40‑inch) are limited to the 6 144‑pixel pipe and thus can’t show 3 840×1 620 even when they support 120 Hz.  By September 2025, users discovered that exporting the Dell U4025QW EDID via BetterDisplay and forcing that EDID for another 5 K2K monitor tricked macOS into unlocking 3 840×1 620 HiDPI—proving Apple uses the monitor identifier to decide which pipe to enable .  Without this hack, only the Dell is “whitelisted”.
• 60‑Hz only monitors remain stuck.  Because 3 840×1 620 HiDPI is available only at 120 Hz, monitors that cannot accept 120 Hz at that resolution (many LG ultrawides only do 60/75 Hz) are permanently limited to 3 072 × 1 296 HiDPI.  Users confirmed that the base M4 mini cannot produce 3 840×1 620 HiDPI at 60 Hz .
• Cabling matters.  Achieving 5 K2K 120 Hz requires DisplayPort 1.4 or Thunderbolt 4 cables that support 8.1 Gbit/s per lane and DSC.  Participants who initially used low‑spec USB‑C cables or monitors with limited ports could only get 60 Hz or non‑HiDPI scaling; swapping to the Dell‑supplied Thunderbolt 4 cable or a high‑quality 8 K DisplayPort cable fixed this .
• Software workarounds are imperfect.  BetterDisplay’s virtual‑screen mirroring can create a 3 840×1 620 HiDPI mirror even on unsupported monitors, but this uses off‑screen rendering, consumes GPU resources and causes high temperatures, and disables HDCP .  SwitchResX can expose hidden modes but cannot overcome the HiDPI limitation for most monitors .
• No complete fix as of Dec 2025.  Apple acknowledged the problem to multiple users and promised a future macOS update .  However, by macOS Tahoe in late 2025, users still reported that only Dell’s U4025QW unlocks 3 840×1 620; other monitors remain restricted and require EDID overrides .  A user upgrading his M4 Pro mini to Tahoe still saw only 3 200×1 800 or 3 360×1 890 HiDPI options .

Summary of limitations:
• The M4 Mac mini’s hardware can drive 5 K2K at 120 Hz, but macOS uses a conservative algorithm that often caps scaled HiDPI widths to 3 072 px (6 144 px source width) unless a monitor advertises 8 K/120‑Hz modes .
• 3 840×1 620 HiDPI is only offered at 120 Hz; no 60‑Hz option exists .  Monitors limited to 60/75 Hz at 3 840 cannot use this resolution.
• Only the Dell U4025QW currently triggers the 3 840×1 620 HiDPI mode without hacks ; other 5 K2K monitors require EDID hacks or virtual‑screen tricks.
• A high‑quality Thunderbolt 4/DP1.4 cable supporting DSC is necessary; low‑grade cables or HDMI ports usually force 60 Hz and prevent HiDPI scaling .
• As of December 2025, macOS “Tahoe” has not fully resolved these issues; Apple appears to have whitelisted only certain displays , so many 5 K2K monitors remain limited.

[waydabber]

Honestly I don't know if it will work or not and the EDID hack helps or not. :) Sorry about that. I'll convert this to a discussion, maybe somebody with a similar setup can chime in.

[lewismoss]

An update on this, I got my M4 macbook air, and I get 3840 x 1620 natively, with full refresh rate available (but interestingly HDR does not work when going above 2560 x 1080. This is without Better Display

[jeremysabath]

Thanks for the info @lewismoss and @waydabber. To help others, here is what I see natively with an M4 MacBook Air (Sequoia 15.6) connected via Thunderbolt to the Dell U4025QW:

I am also going to be testing out the new Dell U5226KW (52" 6k) to confirm we can still get the 3840 x 1620 hidpi there as well, and will report back 🤞

[jeremysabath]

Confirmed that M4 MacBook Air will drive the 3800 HiDPI resolution on the Dell U5226KW

[ayush-lal]

How is everyone getting 3840 x 1620? Im only getting 3360x1418 on my M4 Pro running Tahoe...

[ArturKorop]

I got Samsung G75F for two-monitors setup with old 4K monitor and Macbook M4 Pro, but can't make it work together: either I have 3840x1620 on G75F but laptop doesn't even see the second monitor, or I connect 4k monitor first, but then laptop see G75F as 3440x1440 only without HiDPI options.