The most reliable method currently available is technically a firmware revert
The ability to downgrade a PlayStation 4 (PS4) console to an earlier firmware version is a highly sought-after capability within the console's modding and homebrew community. Primarily, users seek to return to specific "golden" firmware versions—most notably version 9.00 or lower—to enable the execution of homebrew software and custom modifications. While Sony does not officially support reverting to older software, technical methods involving hardware manipulation and specific software tools have made this possible for certain console revisions. The Motivation for Downgrading downgrade tool ps4
Downgrading a PS4 is a complex process often surrounded by myths. Unlike a PC where you can just reinstall an older OS, the PS4 has built-in security measures to prevent this. However, thanks to the modding community, there are specialized "revert" methods that act as a downgrade. The most reliable method currently available is technically
Before attempting to use these tools, several strict criteria must be met: Firmware Revert - ConsoleMods Wiki The Motivation for Downgrading Downgrading a PS4 is
What your is (e.g., playing backups, emulators, or Linux)