Macbook pro M4 to two LG ultra wide monitors

Those 34WR55QK (I could not find 555) are 4K displays (3440 by 1440) with the bottom half missing, and 10 bits/color HDR capable. They must meet the same requirements as full 4K displays.

inputs include HDMI, DisplayPort, and USB-C.

At those data rates, you MUST have high-spec cables shorter than 1 meter

USB-20 trademark or higher for modern USB cables

(HDMI cables must be ULTRA certified, and can can be slightly longer). PREMIUM HDMI cables are slightly lower spec, but will NOT suffice for 4K at 10 bits/color.

Don't count on the cables "shipped in the box" to have any features except MAYBE the correct connectors. These are often "lowest bidder" cables.

ncon64 wrote:

....x 2 LG ultra wide screens (34") 34WR555QK-BL

If Mr. Bennet-Alder is correct, and the model number has only two '5's, those would appear to be these:

LG 34WR55QK-B – 34" UltraWide™ WQHD HDR 10 100Hz Curved Monitor with USB Type-C

34" monitors with a resolution of 3440x1440 pixels, a 100 Hz refresh rate, and support for 10-bit-per-channel color ("HDR10").

They have DisplayPort, HDMI, and USB-C (DisplayPort) inputs. They can deliver up to 65 watts of power to a computer. (If offered several power sources, your MacBook Pro would only charge from one, but would pick whichever one had the highest power rating.)

Use an external display with your MacBook Pro - Apple Support mentions M4 MacBook Pros but does not say that you can get refresh rates higher than 60 Hz on any port except the HDMI port. So I don't know if the Mac would negotiate 100 Hz links to these monitors or 60 Hz ones.

For what it's worth, 3440x1440 pixels @ 100 Hz would be almost exactly the same amount of data as the more common 3840x2160 pixels @ 60 Hz, so a USB-C (DisplayPort) connection presumably has enough bandwidth that the Mac could support the former (if the hardware/software is there to do it).

Which specific LG monitors are these?

Which chip do you have in your MacBook Pro – a plain M4 chip, a M4 Pro chip, or a M4 Max chip? I believe that even the plain M4 chip can support up to two external displays, but it still might help to identify the configuration.

My understanding is that recent versions of macOS do not like to see any transmission errors and will deliberately cut resolution or even signal when they see errors.

• All USB-C cables are not the same. Some don't even carry USB data, just 5V power. You want a cable that is designed to be able to carry video.
• Sometimes the video cables that you get in a monitor box are "lowest price bidder" cables, and not always the ones that you'd want.
• The longer a cable is, the more likely it is to act as an antenna to pick up radio interference / errors, which may trigger the Mac to cut the display signal.

I have no way of telling how good or bad your cables are, but it seems at least possible that your cables might be the source of your misery and that higher-quality cables that aren't too long might resolve the problem.

are your cables labeled CERTIFIED or have a USB trademark {SuperSpeed plus, USB-20, USB-40, USB-80} on on both ends?

If not, try obtaining a different high spec cable as a test.

You could also connect one of your displays with a DIRECT ULTRA HDMI cable (NO adapters). At around 4K, that setup has slightly more bandwidth than USB-C, so would support higher refresh rates at 10 bits/color.

I think that would be 144 Hz HDMI as opposed to USB-C supporting only 60 Hz.

Remember that Readers here are other Users like you. We have no ability to intervene on your behalf, only recommend.

MacBook Pro M4 is NOT an experiment. It works great with the very expensive Apple brand displays, which have been tested extensively.

If you choose to wander off the rose-petal covered path that Apple has provided, and install non-Apple displays of your own choosing (and save CONSIDERABLE amounts of money) that choice comes with some additional work on your part in exchange for that savings. MANY users have been able to do this very effectively, but there is some complexity to it.

If your display makers can not help you make these specific displays work, then THAT is the part that should be sent back. That should be their area of expertise.

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It does not help that that OTHER Operating System uses only the "looks good enough to me" standard for display signal quality.

The Apple standard for its built-in hardware-accelerated displays, makes them suitable for full-motion video for production/display of cinema-quality video with NO dropped frames, and NO dropouts or partial-blank scan lines due to memory under-runs or other issues. Any errors in transmission will cause the display to disconnect, or never connect in the first place.

ncon64 wrote:

They advertise two screens attached to this model without any suggest one might have to get a particular type of cable other than a usb-c …

I'll send the Macbook back on the premise I've been sold an experiment!!!

The fact that USB-C cables vary considerably in capability and quality does not mean that you've "been sold an experiment"!!! USB-C is a connector that can support many things – and a standard for making them coexist if several are present. That's it. It's not a guarantee that every piece of USB-C equipment supports every possible capability.

This is not an Apple thing. It's a USB-C thing. It extends not just to ports, but to cables.

At the low end, there are USB-C cables that just support 5V power transfer and that do not support data transfer at all. There are cables that only support data transfer at USB 1.0 and 2.0 speeds. Then, at the other end, there are Thunderbolt 4 and Thunderbolt 5 cables that are built to handle very high-speed transfers – and can handle just about anything you can throw at them. Cables suitable for USB-C (DisplayPort Alt Mode) fall somewhere in the middle.

Who says that Apple doesn't test with other displays? But I doubt that they test every Mac with every possible third-party display on the market. There are too many of them.

By the way, I am writing this on a Mac that has a third-party display …