Child of Prophecy and that jutsu...........

[zenron]

That is a pretty awesome theory. Well thought out, and I wish it would happen.

Although I think the theory was written awhile ago, it sees pein as the final villain, when he isn't.

 

[Kisuke Kakashi]

In honesty, it makes no sense that Naruto would have had a jutsu that powerful, and not have used it yet, even against Pein.
It would be as laughable if it came to be, as those random Sasuke power-ups.

One of the best things we see with Naruto, is that we tend to experience his "training" with him.
Though it does seem odd that in the two and a half years of training with Jiraiya, he only learned a bigger Rasengan and how to use his clones more effectively...

But what about when fighting Deidara, to rescue Gaara's corpse?
Against Itachi?
Orochimaru?
Why not use "that jutsu"? It doesn't make sense.

The only conclusion I can make is that it was a translation error, and that Jiraiya was warning him not to go four tails...