Evening, all,
At last I have managed to get my Manjaro to understand that I would like to boot into dimension 3840x2160, but it always boots into resolution 96x96 dots per inch.
I am after 160 dpi, but I do not understand how to activate the command [xrandr --dpi 160] automatically… 
Suggestions are very welcome!
Sincerely,
Claes in Lund, Sweden
What desktop environment are you using? Can’t you set it there?
Xfce – and I can set it “per session”. But as soon as I re-boot,
it’ll revert to 96x96.
Ah. I’d put the command into a shell script, then execute the script on login.
Happy for you Claes, that you are making progress! 
I have never understood the Xfce ways of doing things very well … I hope that the suggestion @Mica made re shell script can work for you.
Evening, Old Thing,
Level 3 Pleb, eh? How much more do you need to become a 1st Class Centurion?
No, unfortunately the /etc/profile advice did not work either, nor did my standard invoke-at-start tricks. I presume that Manjaro/xfce is somewhat special.
Presently, I am playing with Manjaro/KDE which looks much more HiDPI-friendly…
[I also tried Fedora 29, but it kindly refuses to boot a Ryzen 7/Crosshair VII Hero combination.]
Ah well…
Have fun!
Claes in Lund, Sweden
Does Manjaro have /etc/rc.local, or equivalent? /etc/profile will only be executed for shells.
No, have not found any; neither in Manjaro/xfce nor in Manjaro/KDE.
In KDE, You can go to Settings > Display and Monitor > Displays, then click the Scale Display button near the bottom. That should let you scale your display.
Agree! KDE seems much more HiDPI friendly!
See KDE paragraphs here: https://forum.manjaro.org/t/hidpi-support-in-manjaro/26088