What is required in order to run dual 4K @ 120Hz monitors on Macbook Pro M4?

What is your reference for the machine supporting two 120 Hz displays? I don’t see anything about that here:

MacBook Pro (14-inch, M4 Pro or M4 Max, 2024) - Tech Specs - Apple Support

Macbook_Pro101 wrote:

What is required in order to run dual 4k monitors at 120hz on a Macbook M4 Pro? Apple says it can handle that but I have yet to figure out a way to do it. I have two macbook m4 pro laptops (13"/15") and I have not been successful with either one.

I have tested various different connectors at this point. Such as USB C to HDMI 2.1 connectors that have the driver flashed to work, HDMI 2.1 to USB C cables, powered USB-C to HDMI 2.1.

I can't get both displays to get signal at the same time. Each one works individually at 4k @ 120hz but two of them won't work together which means its a throughput issue. But the M4 pro is supposed to be able to handle 4k @ 120hz just fine.

USB-C To HDMI (Monitor 1)

Macbook PRO HDMI Port to HDMI 2.1 (Monitor 2)

The newest macOS are increasingly fussy over sub-par cables for connecting external displays.

Often recommended high quality cables: certified Ultra HDMI or Premium ≤ 1.0 m length

or USB-C Thunderbolt5 cables ≤ 0.5 m length cables rated for high speed data...

What make&model displays are you connecting?

Do you need 10 bits/color (HDR) ?

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The following are limits based ONLY on the standards, NOT what the Mac is willing to supply:

USB-C Display data rates top out just under 20 G bits/sec., However, HDMI and DisplayPort both have speeds that increase only in discrete steps.

• HDMI highest step that fits inside a USB-C envelope is equivalent to HDMI 2.0, and runs at 14.4G bits/sec (spread over three data pathways plus a clock signal). Conversion of a USB-C signal to HDMI 2.1 is not reliable or practical.

A 4K display over HDMI (derived from USB-C) can not run faster than 68 Hz. A 4K HDR display can not run faster than 55 Hz.

• DisplayPort highest step that fits inside a USB-C envelope has a much higher data rate, HBR2, of 17.28 G bits/sec (spread over four data pathways).

A 4K display over DisplayPort (derived from USB-C or encapsulated in USB-C) can not run faster than 81 Hz. A 4K HDR display can not run faster than 65 Hz.

Conclusion:

DisplayPort over USB-C (or simply USB-C) connection can theoretically produce slightly faster and more reliable connections than HDMI derived from USB-C

Use of a ThunderBolt port can produce display data rates just under 40 G bits/sec. But Unless your display takes Thunderbolt directly, most practical adapters are not avaIlable as a dongle, and are built into Docking Stations.

The DIRECT HDMI port (in most Macs M2 and later) can produce Display data even faster, at 42 G bits/sec, and supports HDMI 2.1 directly. That is the ONLY known way to get HDMI 2.1 equivalent data rate on ONE cable.

MSI MAG 274UPDF E16M appears to be a 4K display up to 160 Hz refresh rate, with HDR implemented by using FRC inside the display. Picture-in-Picture and Picture-By-Picture supported.

interfaces include:

2x HDMI 2.1

USB-C with 15 W charging ability (too small charging to be really helpful)

DisplayPort 1.4

First display:

One 4K display HDR=OFF on the direct HDMI with a ULTRA HDMI cable can run at up to 144 Hz without loss.

Second Display options:

using a Thunderbolt Dock with a DisplayPort 1.4 output, you could use ONE cable to connect a 4K display with HDR=OFF at up to 144 Hz without loss.

using TWO Cables, you could connect a USB-C cable to the USB-C port, and either:

• a USB-C to DisplayPort cable/adapter or

• USB-C to HDMI adapter/cable

to a second input on the display, and turn on Picture-By-Picture to get both Half-displays back together on the screen. Users who have tried these two cable solutions say that once set up, they work great, as if they were one cable.

if yours is a MacBook Pro with PRO processor, it is limited to TWO external display interfaces, so this Two-cable solution will not work for that Mac.

USB-C cables and cable/adapters are limited to ONE meter maximum.

<< And then a USB C dock with 140 watt PID and HDMI 2.1 input to HDMI 2.1 (3 ft cable) going from the USB C connector on the Macbook to the monitor. >>

USB output from the Mac is limited to at most 20G bits/sec. so as I noted above:

Converting that USB-C output to HDMI can not go higher than 60 Hz.

Converting that USB-C output to DisplayPort, the standards suggest it may support up to 81 Hz. So it is conceivable (but NOT guaranteed) that you MIGHT be able to set it to 75 Hz, but 120 Hz,?? NEVER, when you start with a USB-C output to a USB-C Dock.

A Thunderbolt output to a Thunderbolt Dock fits the standards, but exceeds Apple specs. So that might not work either.

On an M4 MacBook Pro only the HDMI port can support refresh rates higher than 100Hz and there's only one so you can only connect one display at that high of a refresh rate.

Any display connected the Thunderbolt 5 ports will be capped at 60Hz.

click here ➜ MacBook Pro - Tech Specs - Apple

When you choose Retina “like 1920x1080” mode, the Mac is sizing things as if the display had only 1920x1080 pixels, but is actually drawing in detail on a 3840x2160 pixel canvas.

You could go to extra trouble to select “1920x1080 (low resolution)” mode, but there are no guarantees that this would give you the high refresh rate that you seek. (And the Mac might send a 3840x2160 pixel signal to the monitor even in this mode.)

You will not get 120Hz on a second display over Thunderbolt regardless of what you do. It's limited to 60Hz.

With that said, if you set the Mac's resolution to 1080p then that's what it will send, but resolutions are display specific. Setting the built-in display's resolution does not change the external display's resolution.

Also, the display itself can upscale the received resolution to something it can better display, so it may upscaling to 4K or just expanding it to fill the screen. You probably won't get empty space or black bars if that's what you expect.

Re: “using a Thunderbolt Dock with a DisplayPort 1.4 output, you could use ONE cable to connect a 4K display with HDR=OFF at up to 144 Hz without loss.”

Maybe. The OP still has not provided a link to any Apple Support document to back up the claim that Apple says that the Mac supports driving two displays at 120 Hz.

Re: “I tried using the 1920x1080 (low resolution) mode. That does work actually. But the text is super blurry on the screen with that mode.”

That’s because the text is only effectively being drawn with 1/4th as many pixels. So the letter shapes are not as close to the ideal letter shapes as when you are using the Retina “like 1920x1080” mode.