If you use a 4K monitor (3840x2160) but have Windows scaling set to 150%, FL Studio will actually render at roughly 2560x1440. Your 4K image will look zoomed in and blurry.
I can give you exact customization steps or recommend matching color themes. fl studio background image size
For FL Studio background images, the ideal size is 1920x1080 pixels If you use a 4K monitor (3840x2160) but