2 Monitor Setup with 5k Monitors and 120 hz

Both HDMI and DisplayPort (used inside Thunderbolt and USB-C) are packet interfaces. In each case, the interface runs at one of a few Fixed speeds, and if there is LESS data than fully fills every packet at that speed, then packets are (generally) omitted, or stuffed with null data.

You can not run combinations of resolution, bits/color, AND refresh rate that exceed the top speed of the pathway provided. If the display supports Display Stream Compression (DSC) you can get slightly higher data rates (seen as combinations of resolution, bits/color, and refresh rate).

In all cases, use of cables that are sub-standard or longer than allowed can cause the Mac to drop the connection. Comparing to performance under Windows is nonsense, because Windows does not check for errors in transmission.

For standard ThunderBolt cables, the cables MUST be certified and carry the ThunderBolt logo, and be no longer than 0.5 meters in length. ThunderBolt-5 cables are subject to the same restrictions, and provide no additional benefit unless ALL devices involved are genuine ThunderBolt-5 devices. if you need longer cables, much more expensive ACTIVE cables can be obtained (staring at US$125).

The cables "shipped in the box" by MANY display-makers (other than Apple itself) are often 'lowest bidder' cables tossed into the box at the last moment. They are often sub-standard or exceed the length requirement.

that Dell U4025QW appears to be a 5120 by 2160 display (like a 5K display with the bottom quarter missing)

capable of 10 bits/color implemented by 8 bit color panel and FRC dithering and

48 to 120 Hz refresh rate.

interfaces include:

HDMI 2.1

DisplayPort 1.4

2x ThunderBolt-4 delivering up to 140Watts power.

it also supports Picture-in-Picture and Picture-By-Picture, but

does not appear to support Display Stream Compression (DSC)

using DisplayPort or DisplayPort-over-Thunderbolt-4, that display does NOT appear to be able to achieve 120 Hz refresh using using 10 bits/color -- it exceeds the bandwidth by 17 percent, but works fine at 8 bits/color.

However, the adapter/cable you are using exceeds the length limits four times over. At these speeds, DisplayPort to ThunderBolt cables need to be 0.5 meters or shorter. the way to get more length at this speed is to use an ACTIVE Thunderbolt cable, (starts at US$125).

Over HDMI, you could achieve 120 Hz at 8 bits/color, or about 100 Hz at 10 bits/color

THIS display should be connected to the Direct HDMI port, without using any adapters, and longer lengths can be used, likely up to 8 feet PROVIDED you use the appropriate HDMI cable.

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HDMI cables you want for HDMI-only Displays (higher resolutions than 720p TV sets) are marked as Certified with an anti-counterfeiting tag and are labeled:

"PREMIUM High Speed HDMI cable" or that + "with Ethernet" (up to 4K at 30Hz) --OR--

“ULTRA High Speed HDMI cable" or that + "48G" (supports higher resolutions and backward-compatible)

Cables with No Certification tags are good for your standard 720p TV set, and not much more.

HDMI was invented for HD TV sets. it works great at its original resolution of 720i or 720p. At higher resolutions, it quickly develops issues that are complex to solve, and the cables and adapters required to solve are NOT intuitive.

<< (the 5k monitor supports 5k resultion with 120 hz over the provided Thunderbolt 4 cable delivered by the manufacturer >>

it is possible, but EXTREMELY unlikely, that your vendor-supplied Thunderbolt cable is either 0.5 meter max, or is an ACTIVE Thunderbolt cable. if either of those are true the cable should be clearly and strongly label as such. ()Thunderbolt symbol and the word 'ACTIVE'.)

FAR more likely is that the cable they threw in the box at the last moment isa "lowest bidder" cable -- just good enough to keep you from returning the display the same day you got it.

NB>> If you can not verify your cables on hand meet the standards I have described, you need some better cables.

<<. So I thought it is maybe a limitation of the bus traffic and so I thought if I limit e.g. the refresh rate of the smaller one, than the big one will support full resolution and refresh rate. >>

That is an interesting idea, but is NOT the way the M-series Macs work.

There is no pooling of Bus bandwidth,

But nor is there a shortage of Bus bandwidth that would make pooling necessary.

• ThunderBolt ports are limited to the maximum speed of a Thunderbolt Bus, 40 G bits/sec.

• HDMI has made some odd decisions along the way. There is a large gap above higher than USB-C bus speeds. The next quantum step is HDMI 2.1 speed of about 42 G bits/sec, which Apple decided to support for one HDMI Direct port in Macs AFTER the M1. But HDMI 2.1 requires different cables.

On a Mac, if display data transmission errors are detected, the display is dropped.

By far 'the most vexing display problems users are reporting here' are solved by getting their cables up to high enough specs and down to the length limits required for error-free operation at these mind-boggling display data speeds.

When you say that you have a M4 MacBook Pro, which M4 chip is in that machine?

Is it a plain M4, a M4 Pro, or a M4 Max? Even the specifications for the M4 Max model do not promise the ability to run a 5K display at a refresh rate of 120 Hz.

If it is a plain M4, the machine=

"Simultaneously supports full native resolution on the built-in display at 1 billion colors and:

• Up to two external displays with up to 6K resolution at 60Hz over Thunderbolt, or one external display with up to 6K resolution at 60Hz over Thunderbolt and one external display with up to 4K resolution at 144Hz over HDMI
• One external display supported at 8K resolution at 60Hz or one external display at 4K resolution at 240Hz over HDMI"

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

if you can respond with the make&model of your displays, Readers can elaborate about what resolutions, color depth, and refresh rates are possible for each display.

In every case, your cables must not be sub-standard or over the allowed length, or the display may drop out due to transmission errors.

the Asus ROG SWIFT PG278QR appear to be a "2K" display at 2560 by 1440 capable of 8 bits/color and

refresh rates from 24 to 165 Hz.

interfaces include:

HDMI 1.4

DisplayPort 1.2

.

Because the display does not implement highest data rates from more recent standards:

the highest possible under HDMI Direct or derived from USB-C is 147 Hz.

the highest possible under DisplayPort, Thunderbolt, or USB-C is 165 Hz.

Adapters and adapter/cables involving DisplayPort at these more modest speeds are likely limited to ONE meter, maximum. this is also the top spec for USB-C cables and cable/adapters.

what make&model display(s) ?

what sort of work are you doing? Is 10 bits/color important to you or not?