Thank you for reporting a bug to Unity.
We have reviewed the issue carefully, and in this case, the team is unable to proceed with a fix as the reported behavior is expected. Specifically, the superSize argument in CaptureScreenshot is designed to increase the pixel dimensions of the output image file rather than modifying the engine's internal rendering resolution. Because of this, a 4x superSize output is not expected to match the fidelity or visual quality of a native 4K resolution screenshot.
There are a number of reasons we make this decision, including the impact and severity of the issue across our user and customer base, and the possibility that future plans may solve the problem in a different way, or that a workaround for the bug may be available.
Today we will be closing this case. Thank you again for taking the time to report this issue, and please let us know if there is anything else that changes the impact or severity of this issue.