Be careful HTBH, Scoop doesn't quote all of the requirements for PS off rotation deadhead in Section 8.E.8

So in your situation, yes, a backup is required because the original rotation had a backup, that is, a subsequent flight. See 8.E.8.e:

"e. the routing provides for a subsequent flight that is scheduled to arrive at a reasonable time before his required report under Section 8 E. 5. if the pilot’s originally scheduled routing provided for such a subsequent flight."

If you do as Scoop suggests then you fall under 8.E.8.e Note two:

"Note two: A pilot who utilizes an off-rotation deadhead at the beginning of a rotation that does not satisfy the requirements of Section 8 E. 8. will be responsible for his transportation."

That means if you do as Scoop suggests and take the last possible flight then you are not entitled to PS and you are on your own. You will have no leg to stand on if you don't show up one hour before your working flight. But hey, maybe grandma is driving you to the airport or something like that. I hope so.

Anyway, because in your original premise you have five flights... you could do as you propose and move to your DH to the fourth of five flights as long as you can successfully book PS within 14 days for that fourth of five flights and the subsequent (backup) fifth flight arrives at a reasonable time before the one hour report of your working leg.

That's how I see it anyway. Be careful out there.