If I load the 4K image into three I get jagged / pixelated looking background. If I use the same image in blender I get much better results. Here it is in blender
i believe RGBELoader & CubeTextureLoader limit the resolution as the primary function of the background (at least from what i thought) isn’t to show something but to light the scene realistically and performantly. you can always just use a sphere/boxGeometry side=BackSide and project the texture in any resolution you want, or film it with a cube camera into a webglrendertarget whose texture you can stick into scene.background again.
TBH, I can’t see a noticeable difference when using the 4K equirectangular HDR texture in the three.js editor as Scene.background compared to the Blender setup.