You've probably downloaded the smaller update file.
You need the one at the bottom of the page https://www.playstation.com/en-gb/get-help/ps4-system-software/?cid=system-software-update-three-column-01-ps4-eu-01may15
in the section called "Perform a new installation of the system software"
Delete the smaller update file from your pc or download to a different location so the new download does not get renamed.
Be aware, this will delete everything from the hard drive, but if the system is asking for it, there isn't anything you can do about that.