M4 Pro MacBook Pro unable to connect two external displays

DisplayPort cables and USB-C cables for use with displays are limited to ONE meter maximum length when used on a Mac. Any longer and there is substantial risk of transmit errors due to signal degradation, and the display may drop out.

ThunderBolt -3 or -4 or -5 cables are limited to 0.5 meters maximum length. Longer ACTIVE cables can be had, but they are Much more expensive (starts at US$125).

it is possible the special ThunderBolt-5 cable "shipped in the box" with the Dock is capable of longer lengths, by design. However, most "shipped in the box" cables that come with displays are 'lowest bidder" cables, and not usable at the highest settings.

Your M4 MacBook Pro with PRO processor is Thunderbolt-5 capable.

NOW I see what is the fundamental problem.

That CalDigit Dock only has one real DisplayPort output:

There are lots more outputs, but these would need additional adapters or adapter/cables, and as I said above, everything I know of can only do USB speeds, not Thunderbolt.

It looks like the best solution might be a direct HDMI cable for one display, and the Dock for the other, with an appropriately short DisplayPort cable.

<< I didn't realize that Macs had as severe limitations as this when it came to the cables used. >>

The Apple standard for its built-in hardware-accelerated displays, makes them suitable for full-motion video for production/display of cinema-quality video. Folks really are producing commercial feature films on Apple equipment, so It MUST meet the highest standards. That can not tolerate any dropped frames, and NO dropouts or partial-blank scan lines due to memory under-runs or Transmit errors due to low-spec or overly long cables. 

These acceptable settings are obtained by a query to the display itself. In most recent versions of MacOS, no transmit errors are tolerated, or the display will drop out. 

Under Windows, you manually choose the settings you prefer, or download a popular "Driver" (which is actually a package of resolutions and setting thought to be good ones).  Whatever happens is your problem.

<< it showed 2 meters for the USB-C to DP at 48Gbps and up to 3 meters for the DP to DP at 48Gbps >>

Neither USB-C nor DisplayPort alone can run at 48 G bits/sec. The upper limit is just under the maximum USB-C 3.2 speed of 20 G bits/sec.

I am not certain where you got information about cable lengths. There may be lots of folks running 'that other Operating System" with displays with microscopic random snow in their pictures, and they are perfectly happy.

NOT on a Mac. The Mac has 'no errors' as its standard.

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The signals used for DisplayPort and USB-3 Volts use dual-rail balanced drivers and receivers. It is conceivable that when driving only one display-cable, you 'get by' with so little noise that the signals squeak through most of the time. That does not indicate they are Robust enough for long term use. Adding an additional display might add enough additional noise to make a margin connection Into a connection that incurs errors often enough to drop out.

Re: “Neither USB-C nor DisplayPort alone can run at 48 G bits/sec.”

This might depend on the DisplayPort version (e.g., v2.1 vs. v1.4 vs. v1.2), and on whether the USB-C connection is simultaneously carrying data at USB 3 speeds.

https://kb.plugable.com/docking-stations-and-video/understanding-displayport-21

The linked site suggests that a USB-C ( DisplayPort 2.1 ) connection can support up to 80 Gbps if four lanes are available for carrying video data.

At the higher speeds, it might be harder to achieve error-free transmission with long cables.

ThunderBolt-3 or -4 connection:

if you had a display that could accept a direct ThunderBolt -3 or-4 connection, it would provide an encapsulated DisplayPort 2.0 or higher top connection speed of UHBR3 at 38.69 G bits/sec, which fits inside the 40 G bits/sec Thunderbolt-3 or -4 envelope.

That could achieve 4K at 10bits/color at 144Hz refresh rate, and likely 180 and 240 with Display Stream Compression. But its cable might be limited to 0.5 meters maximum, unless you used an ACTIVE cable (much more expensive, starts at US$125).

DisplayPort Dongles and adapter/cables:

All the DisplayPort dongles or cable/adapters I know of are actually USB-C connected devices, not Thunderbolt. Therefore, these are limited to the DisplayPort speed step that fits inside a 20G bits/sec USB envelope, HBR2 at 17.28 G bits/sec, and can only support a 4K display 10 bits/color directly at up to 60 Hz, likely higher with Display Stream Compression, but NOT 240 Hz without color changes. at 8 bits/color, they can directly support 75 Hz without Display Stream Compression, but again not 240 Hz without color changes.

Cables for this use are limited to ONE meter maximum when used on a Mac. The only exception would be if you used an adapter in the middle of two cables. Then you could have one meter of USB cable leading in to the adapter, then one meter of DisplayPort cable, leading to the display, for a total of two meters.

<< 27" LG UltraGear G8. >>

that is not a complete model number, but it appears to match this 27-in LG 27850-A

a 3840 BY 2160 (4K) display capable of HDR 10 bits/color and refresh rates 48 to 240 Hz.

supports Display Stream Compression and possibly Picture-By-Picture

connectivity includes:

2x HDMI 2.1

1x DisplayPort 1.4

When the first Apple-silicon Macs with HDMI ports were released, the supported HDMi was version. 2.0, which fits inside a Thunderbolt-3 or -4 envelope of at most 40 M bits/sec. Since HDMI is packetized interface, and only runs at certain overall data rates, the fastest was HDMI 2.0 top speed of 14.4 G bits/sec.

At that overall data rate, a 4K HDMI display at 10 bits/color could only achieve 50 Hz refresh without changing the color format or dropping back to 8 bits/color and losing numbers of simultaneous colors.

Apple engineers set to work and made the display generator that runs the HDMI port compatible with the new signaling changes and faster speeds required by the next faster step -- HDMI 2.1. Apple-silicon Macs with M2 PRO and everything in 2023 and later supported HDMI 2.1.

That means one Hardware display generator (the automaton that fetches the bytes of an active screen buffer, and serializes them to be sent to the display 60 or more times a second) runs the DIRECT HDMI port at about 42 G bits/sec, slightly faster than top Thunderbolt-3 or -4 top data rate of 40 G bits/sec for all others.

Using HDMI 2.1, a 4K display at 10 bits/color can achieve 144 Hz speeds directly, and likely 240 with Data Stream Compression

The footnotes say, ULTRA cables required, and NO adapters allowed at these high speeds.

Thunderbolt-3 allowed connection to One thunderbolt docking device, but only supports daisy-chain to additional devices. 40 G bits/sec top speed.

Thunderbolt-4 allowed the introduction of a composite data stream servicing multiple Thunderbolt devices on the same cable. 40 G bits/sec top speed.

My expectation is that Thunderbolt-5 can support MORE displays, but perhaps not higher data rates for each display, YET. Unless/until Apple revises upward the display generator speeds, we are limited to at most DisplayPort UHBR at 38.69 and HDMI 2.1 at 42 G bits/sec.

The underlying signaling rate for Thunderbolt-5 is no different that Thunderbolt -3 or -4. What changes is that Thunderbolt-5 uses a modulated signal, now packing up to three bits data into each signaling interval. This can increase the speed to double or triple the 40 G bits./sec. But each and every device and cable must be ThunderBolt-5 to achieve this speed bump.

Cable length requirements are the same for all. For error-free transmission display data on a Mac, 0.5 meters maximum for ordinary passive cables.

It is not lost on me that 0.5 meters barely reaches from center to side of some displays current available.