[Zanoan]

I was always under the assumption that the Fourth used the technique the Third used on Orochimaru, back when those two had the fight on the roof when the Sand Village and others invaded Konoha. (Third didn't succeed it fully due to his injuries but obviously took atleast Orochimaru's arms resulting in him being out of the game for quite a while.)

The technique requires the sacrifice of the user itself though, where the user and whoever is "consumed" will die and forever struggle in the stomach of that Death God or whatever it is. In the case of Naruto, the Fourth and the Nine-Tails, it would seem that it - somehow due to the excessive powers of the Nine-Tails, required a temporal seal as well, being Naruto and still the sacrifice of the Fourth of course.

But I could be wrong. And it was just until a few recent chapters that they mentioned that Naruto was in fact the outcome of the Fourth - I don't think they mentioned that before? But then again I never read the mangas from the start. Only watched the anime. Got too impatient with Shippudden and decided to read up until from where the anime stopped.