Downgrading Firmware
Firmware Revert
By dumping with hardware Syscon memory and flash memory of a specific PS4 console, one can update the System Software of this PS4 to any version then whenever he wants, restore the dumps in order to restore the older firmware. Then it is required to either restore a HDD dump from that Firmware or to reinstall Firmware with PUP.
Actual Downgrade
There may be a way to downgrade the firmware without need of a hardware backup. This downgrade might be infeasible due to per-console keys and unknown crypto keys used to decrypt PUP and re-encrypt respective flash components. That would require full code execution inside SAMU, or at least keys from SAMU.
Current Firmware Version Modification or Bypass
SNVS modification
Current Firmware version is stored in Syscon SNVS. SNVS is an area encrypted with per-console keys. SNVS encryption is handled by SAMU.
See the research done by fail0verflow on PS4 Syscon. SUCK MY DICK SUCK MY DICK SUCK MY DICK [1]