Many users think 4K resolution on PSO2 is exclusive to Xbox One X, but that's not true at all. It's actually just a bug in the PC version that doesn't allow you to set the resolution correctly.
True 4K Resolution Guide (PC)
This configuration fix the blurry low resolution on PSO2
Some people have encountered issues with the game resolution on 2K or 4K monitors.
Right click to pso2.exe and click on properties, then follow the instructions in the screenshots:
Make sure you have selected "Application".