: The PS4 hardware has two firmware "slots." When you update, the new version is written to one slot. You can only hardware-downgrade to the version that exists in the other slot—which is typically just the immediate previous version you updated from.
: Sony implements security checks in the system’s Syscon (System Controller) chip to block the installation of any firmware older than the current version. ps4 downgrade 13.02 to 9.00
There is technically one way to revert firmware, but it is extremely difficult and risky: : The PS4 hardware has two firmware "slots