I doubt it's HDMI cable.
I had some doubts at first hence bought other 2 RPI4s believing the prior one(s) to be defective. So here's what happened.
At work I have a need to monitor multiple charts at once LIVE. So we bought a 4K display and a rpi4 to drive the dashboard. Popped in a copy of the latest Raspbian Buster and fired it all up.
At first all was nice and dandy, 2 or 3 days later the magic started to happen.
Out of the blue network all the configuration disappeared and I no longer could bring the network up (P.S. I was using WIFI connection and a BT keyboard). Every time I tried to connect to any WIFI AP I got the same weird IP address that did not conform to either of the networks. Routing table was empty, adding routes manually did not bring the network up. Tried multiple restarts as well. In the end decided to reflash the image and start all over again. That's the first thing that was weird and not sure whether in any way relevant to your topic, but.. maybe?
The new image was also working perfectly for a few days. After that something new started to happen - at some mornings when I came in to the office I used to find the RPi4 disconnected from WIFI. Reconnecting back to the network did work most of the times - the other times I had to restart the device (sudo shutdown -r now). That happened a few times. Still connected to the same 4K display.
Later on something new started happening: the BT keyboard could no longer connect to the RPi. Multiple attempts to rfkill the bluetooth device did not help (or maybe did, just not enough?) - the keyboard still was not able to connect, although sometimes I could see the cursor move a few milimeters (the keyboard has a trackpoint). Restarting the RPi "fixed" the bluetooth thing every time.
We got another RPi hoping that one was defective. The new one had the same issues too. I brought the old one home hoping to do some tests and identify whether it is OS or HW issue. I hooked up the serial connectors and connected to RPi with my laptop. No display connected (I do not have HDMI displays). I set up WIFI connection to my router and connected via ssh for more convenient access. After a while I walked away to have lunch and came back to find a frozen ssh connection. Hooked up UART adapter again to find that WIFI got disconnected, wlan0 got the same weird IP address and an empty routing table. Could not reconnect to my wifi router without a restart. Let me repeat myself: NO display was connected to the RPi. Before restart I saw some errors (bcm? Something I translated into Broadcom at that moment) with an error code -5<something> (-52? -53? Can't remember). Restarted the device and the network came back up. Could not reproduce that error ever since. But I saw it at the RPi in the office a few days later (w/ 4K display), when BT failed again.
[not in chronological order] another weird issue was happening with all the RPis and that was the reason why I bought the 2nd RPI4. As I was setting up the first RPI4 I did not yet have the 4k display, so I was using my 1920x1080 monitor. All the set-up process went fine, then we got the 4k display. Plugging the rpi4 to the display was troublesome - it could not stretch out the image to 4k. Reflashed the sd card and rebuilt the dashboard on the 4k display this time. All went peachy. Then plugged in my monitor (can't remember the reason why tho) and all I could see was black. Not even the RPI logo was seen when booting up. Nothing. Only black. But the OS booted up as I saw new records in syslog later on. Tried messing with boot parameters in boot.conf (?) - didn't help. Since then even 4k randomly does not work. Reflashing the sd card again did fix it. Apparently RPi4 (or Raspbian Buster?) does not like to be switched between different resolutions, if either of those resolutions is 4k. If I set RPI4 up using 4k only - it works well and is stable (image-wise). If I try to connect it to any other display - video stops working randomly on any displays you are to connect the RPi to from that moment on, until you reinstall the OS. But that was the point when I ruled my first RPi defective. That was a wrong move as other 2 RPI4s I got (all from different vendors) had all the same issues
Not sure whether my story has any value in this context. Maybe it could suggest it's not the HDMI cable to blame network issues. Maybe it has something to do with internals of the board as I got network outage even w/o any video cables connected. Maybeit has something to do with on-board graphics, as they are obviously misbehaving. And maybe it's just an OS issue since after reinstalling the OS things get back to normal. What's still bothering me is that I still have the first RPI4 at home connected and booted up waiting for network to go down again to have a proper post-mortem analysis. Could not make it happen - it's been a few weeks now.