Hi ! I have a 4K monitor and when loading and changing the user (when entering the login), the screen resolution constantly changes. How do I apply one resolution for all users ?
Hi and welcome to the forum 
If you are using GDM as login manager this wiki page will help you copy the user settings to the GDM user. The GDM user then has the same settings and the resolution switching should not occur anymore.
Thanks. Does this work in both Xorg and Wayland ?