Unlike newer firmware (10.00, 10.50, 11.00, etc.), became famous because exploit developers (like TheFlow) released a jailbreak for it. This allows advanced users to run homebrew software, backup their own games, and use custom mods.
If your PS4 is already on 10.00 or higher, you cannot downgrade to 9.00. Sony does not allow firmware rollbacks. ps4 update 9.00 firmware download