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Mine does this occasionally and this is with a very decent HDMI cable. It's more of an annoyance than a real problem.
 
Same here, yesterday but only one time, I use a hdmi 2x1 switch to toogle my monitor between my windows work laptop and the Mini M4
 
Get a new cable

It's an issue with the actual ports. And again it's not limited to just the Mac mini as my Dell Micro does the same thing. I have personally tried many different HDMI-HDMI cables from the ones that came with my HP and Acer monitors along with many different brands of quality HDMI cables.

I am using HDMI to DisplayPort cables with my Dell since that is all it has. But the monitors still blank out just like the Acer and HP monitors on my 2018 and M4 Mini's when using HDMI-HDMI cables.

No amount of swapping cables helps. The only thing that helps with the Mini's and the Dell Optiplex Micro is using HDMI - USBc cables.
 
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It's an issue with the actual ports. And again it's not limited to just the Mac mini as my Dell Micro does the same thing. I have personally tried many different HDMI-HDMI cables from the ones that came with my HP and Acer monitors along with many different brands of quality HDMI cables.

I am using HDMI to DisplayPort cables with my Dell since that is all it has. But the monitors still blank out just like the Acer and HP monitors on my 2018 and M4 Mini's when using HDMI-HDMI cables.

No amount of swapping cables helps. The only thing that helps with the Mini's and the Dell Optiplex Micro is using HDMI - USBc cables.
Some people have fixed it with new cables, whereas many others have not, but you'll never know unless you try it. I was responding to someone saying they had used a "decent cable", whatever that means.

BTW, this is also an issue with my 3rd generation Apple TV 4K on one TV setup. My 1st generation Apple TV 4K works fine with the same cables and same TV setup.
 
I never had an issue with my 2010 Mac mini when using the HDMI port. The same monitors with the same cables caused issues with the 2018 mini and M4 Mini.

My Dell has 3 DisplayPort's and one USBc 4 port. The monitors have HDMI ports. So I use HDMI to DisplayPort cable on three monitors and a HDMI to USBc cable on the fourth monitor. The three connected by HDMI-DisplayPort blank out just like monitors connected to the 2018 and M4 Mini's by HDMI-HDMI blank out.

And yes YMMV when it comes to the HDMI issues. Some have no problems, others have screen that go black for a second or two and others have bigger issues. I will say that the M4 Mini has less issues than the 2018 Mini
 
I'm getting a new LG 6K monitor. I was thinking of using it with HDMI 2.1 instead of Thunderbolt 4, but we shall see if it runs into connectivity issues with HDMI.

BTW, my M1 Mac mini and my M4 Mac mini both worked fine with my Huawei MateView 28" monitor over USB-C... until macOS 26 Tahoe. After I installed Tahoe, the Mac mini does not wake up the monitor after it sleeps. Irritating.
 
I get this everytime I use my M4 Mini and it's incredibly annoying. Almost always a 2 sec black out. I've tried 3 different HDMI cables that I already had and no luck.

I'm considering exchanging it at Costco for another unit but only if the new one won't do this anymore.
 
Hi there, little late in the game but hope my comment helps at least for the state of mind. I got a Macbook Pro 14 M4 base model in July. I got a not so cheap LG 4k 60Hz monitor, it is a very quality monitor, it is even almost match my Macbook's display colors. But this screen goes black for a few sec like 2-3 sec thing, well I have it too. I tried the factory HDMi cable what comes with my montior, I bought a quality 8K Rode HDMi Cable. Same thing happend few times. Finally I bought a 2.1 Super High Speed HDMi 10k UHD 120Hz (its is even got Ethernet in it) from inakustik, very high quality, it was way over my budget back then but I wanted to solve this phenomenon. For weeks it was OK, no problem at all, no blackout. But after MacOS26 update the phenomenon came back. My monitor even flashes between orange and normal sometimes, like when you turn on and off TrueTone. What is interesting in this matter and I don't really saw anybody talk about this, but you can reproduce this "screen goes black for a 2-3 sec" thing in System Settings when you change your refreshing rate of you monitor, in my case from 60Hz to 50Hz or any other then back to 60Hz. Exactly the same thing happens every time you change the refreshing setting value. I also think this is not a hardware issue at all. This is some kind of software or HDMi communication issue between the Mac and monitors or cables. Kinda like it is refreshing the connection or communication but it goes black at the same time. If there is a hardver problem then it would get worse and worse but this isn't the case. Worth mentioning again that this thing happening with me on my M4 MacBook not even with MacMini. Sometimes nothing for a few days/weeks sometimes happen once or twice, and before Tahoe nothing at all. It is more of an annoyance or a phenomenon than a real hardver issues or fault.
 
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