Are you playing it in virtual fullscreen mode?
I remember reading something about there being a glitch where PSO2 can't update resolution values after running it the first time, I forgot where I read that.
I remember the fix being to delete the userfile, run the launcher again, set up your resolution and pick windowed and not virtual fullscreen. Make sure to set the UI scaling correctly.
Launch the game, close it, then change it back to virtual fullscreen and it'll be fine...
Of course, the fix could be something as easy as going into the file in the first place and making sure everything is set to 3840x2160 because some values don't update on their own.
Saw some stuff about that here: https://www.reddit.com/r/PSO2/comments/grml9t/fix_for_game_stuck_in_720p/
If I find the proper post about all that crap I typed up there, I'll link it here too later.
EDIT: Ah, here it goes!