It is apparently possible to revert your PS4 to its previous firmware version, without any backup - Wololo.net
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Watch this, How to Revert PS4 Firmware and Explanation. - YouTube
How to "downgrade" a PS4: full tutorial by Modded Warfare - Wololo.net
Ps4 Downgrade via BWE (Orignal Syscon Write) - YouTube
How to "downgrade" a PS4: full tutorial by Modded Warfare - Wololo.net
PS4 Aux Hax: Hacking Aeolia, Syscon and DS4 by Fail0verflow! | Page 3 | PSXHAX - PSXHACKS
PS4 Downgrade Firmware 8.00/7.55 To 6.72 | Playstation 4 7.55 To 6.72 Downgrade Tutorial - YouTube
How to downgrade a PS4 firmware 8.03 or higher to 6.72 or lower - Quora