Could I steal the elf files from RPi OS and use them on my LE11 nightly, or is this the stupidest thing you've heard today?
I would say it's not an absolute nonsense - it might work but of course it's not guaranteed. All versions (bootloader, firmware and OS) should match together (to be at supported levels). You can test it or wait, I suppose that LE developers are going to upgrade the RPi 4B's firmware as well but it may take some time.
IIUC, the RPi OS that worked well was using the Tue 25 Jan 14:30:41 UTC 2022 (1643121041) firmware.
This is NOT a firmware, it's a bootloader, stored in RPi 4B's EEPROM. Until it's upgraded/downgraded automatically by OS or manually, it stays there even you boot from another microSD card or HDD.
The firmware files are stored on boot media's partition (.elf files etc.). So they are different with every boot media used.
I don't know which firmware version uses latest LE Nightly but you can compare them with files on RPi OS which works for you.
This one is apparently a brand new revision number: b03115
I am afraid that could be a reason... 
Looks it's too new and there are several different (not LE related) issues yet reported with this HW release. It will take some time.
Thanks for the hint, I'll give it a try.
Several months later I have to say this issue is still here, still experiencing it on latest LE 11 nightlies (20220130).
Also the recommended command echo 0 >/proc/sys/vm/oom_dump_tasks does not work (to disable long huge disk activity after OOM killer stops Tvheadend server).
The issue usually hapens when I get back with Timeshift and keep watching the same TV program for a long time. Especially when the Video stream has a higher bitrate. The memory gets exhausted, soon or later, but more or less randomly, despite the Timeshift Maximum RAM size reduction. For some (usually even quite long) time the free RAM is quite high, oscilating about bottom limit (based on Timeshift Maximum RAM settings) but suddenly it gets exhausted quickly to zero.
I suppose it's a Tvheadend / Timeshift issue (currently using addon Tvheadend Server 4.3 (Alpha) 10.80.4.103 but the same with 4.2).
The worst and most annoying issue is that the LibreELEC is completely frozen for unpredictably long time after OOM killer's action (can take hours - tested). It depends on RAM size so I suppose the dumps are flushed to boot media. Is there any way to stop it please?
It's not obvious what you are trying to do / which creator do you try to download. I don't see any issues with file(s) download.
Try the direct download links (I suppose you are using Windows):
Open, download & run:
https://releases.libreelec.tv/LibreELEC.USB-SD.Creator.Win32.exe
or
Open, download, install & run:
https://downloads.raspberrypi.org/imager/imager_latest.exe
Then use the selected app to download & write the image to your microSD card or an USB flash key.
You can also download the LibreELEC image file for RPi 4 manually:
https://releases.libreelec.tv/LibreELEC-RPi4.arm-10.0.1.img.gz
Sometimes the most simple solutions are the best... I would use a black adhesive tape. 
Hi
Maybe better with TvH 4.3? Is there a way to update without loosing configuration? This was much work. I miss the more easy and more user friendly configuration of my old VDR, especially the channellist.
In fact you won't loose any Tvh 4.2 configuration. You can install both plugins, Tvh 4.2 and Tvh 4.3 and enable just one of them. They have their own folders so there are two independent configurations. So as mentioned, you can play with configuration copy from Tvh 4.2 to Tvh 4.3 and get back to Tvh 4.2 easily if necessary.
Kodi is already part of LibreELEC image.
Download the LibreELEC USB-SD creator and LibreELEC image file for Raspberry Pi 4B and write the the image to USB Flash or to micro SD card with LibreELEC USB-SD creator.
is there a way to alter the brightness of subtitles only when watching HDR? in a dark room its almost impossible to have subtitles enabled.
thanks for any help
Did you try to change the Opacity parameter in Kodi Settings - Player - Language menu?
Another solution could be to use irexec and map a script to some IR remote control key. The configuration is very simple and it works great.
Hopefully you won't brick your RPi 4B (even though you install a bad bootloader and your RPi 4B does not boot, it's possible to create a recovery micro SD card which can fix it) but I think it won't help you.
Please note the firmware (which is located on LE filesystem) must support the RPi 4B hardware release. As RPi 4B v. 1.4 is newer than LE 9.2.6 build (and there are significant HW differencies between old and new RPi 4B revisions), it probably won't work with new RPi 4B as the firmware in LE 9.2.6 is simply too old. So you may try to replace the firmware files on LE 9.2.6 filesystem but who knows if it ever helps...
Instead I would upgrade the bootloader to latest stable (pieeprom-2021-12-02.bin) and run LE >= 10.
I wouldn't do it but it's up to you... Try this:
cd /tmp
wget -O pieeprom.bin -c https://github.com/raspberrypi/rpi-eeprom/raw/master/firmware/stable/pieeprom-2020-09-03.bin
rpi-eeprom-config pieeprom.bin > bootconfDefault.txt
# default config
cat bootconfDefault.txt
vcgencmd bootloader_config > bootconf.txt
# current config
cat bootconf.txt
rpi-eeprom-config /tmp/pieeprom.bin --config /tmp/bootconf.txt --out /tmp/pieeprom
rpi-eeprom-update -d -f /tmp/pieeprom
reboot
#Please note you may need to change the bootloader config as there're some changes in recent bootloader revisions.
Some IR remote controls generate two different scan codes for the same key (when you repeat pushing it). Check with ir-keytable -t command if that's your case. Then you need to add both scan codes for the appropriate command to keymap file (so it should contain two lines for each command).
I am not using it but perhaps you could try to search and read something about Audio Profiles:
The same issue on LE 11 Nightly 20220106.
It's very annoying when the Audio is lost and can't be restored without losing Timeshift data.
You need to enable the appropriate Kodi settings:
Services - Control - Allow remote control via HTTP option