In GRUB I could set a gfxmode, but how do I do this on my RPI400? Also, my RPI400 is connected to a TCL 50" TV and always wants to start in 4K, but is unreadable. I have to change the res to 1920x1080 via a script. Is there a way to keep these settings in XFCE?
Try changing parameters in config.txt.
I haven’t tried this out. My Raspberry Pi 4b works well on 4k 50" TCL TV. Maybe try KDE Plasma or other DEs
Does this mean that Settings/Display doesn’t work for you in XFCE? I run (many) XFCE instances (though not on a pi ATM) and several require downshift to 1920x1080 on boot (4K TV also), which has never been a problem…
XFCE display in settings manager does work and if I logout with this setting upon logon it will be 1920x1080. However if the screen locks upon unlocking the screen reverts back to 4k and I have to either use XFCE display or easier xrandr -set 1920x1080. The tty is still 4k and (I think but haven’t checked) gdm is 4k. I would like to be able to set the tty like in grub where you put GRUB_GFX=1920x1080 and initrd video=1920x1080, but does this go in the /boot/cmdline.txt file? And as for gdm I assume it is either a /etc/x11/xorg.conf.d/20-display.conf and setting it there or in the gdm.conf file.
AFAIK, if you set it in Display (with a file name for the saved parameters, usually ‘monitor’) it SHOULD be handled from there. If not, the the same applies to running arandr and saving the file as above. Haven’t played with it in a while (once I got it behaving) so this is a bit vague… something like 20 years ago is clear, but yesterday? 
)
Let me know if I need to research again…
This works up to the DM (SDDM or LightDM) which switchs to 4K. Then LXQT on initial boot remembers the 1920x1080 resolution, but coming back from locked screen it does not remember. XFCE does remember the 1920x1080 on boot and after lock. I just need to check on how to fix the DM.