4th Hokage
The Fouth Hokage was the one who used the Shinigami technique to seal the spirit of the Nine Tailed Fox inside of Naruto. He could not have sealed the Kyuubi into himself because the Kyuubi’s soul could not be contained by that jutsu. The jutsu is designed to take both the caster and the target’s soul and entwine them inside the shinigami, to battle for all eternity. Kyuubi was too strong and would have gained strength while in hell until it was strong enough to overpower the gatekeeper and escape.
And so The Fourth Hokage designed the seal on Naruto to transfer the soul he ripped from Kyuubi’s body into Naruto. The seal on Naruto slowly drains Kyuubi’s Chakra and infuses it with Naruto. And as Kyuubi mentions to Naruto, if Naruto is to die, the Kyuubi is destroyed, with no chance of escaping.
The Fourth Hokage was also know as the Yellow Flash. He was called this because of his technique "The Flying Thunder God", which was more or less a teleportation jutsu like the Body Flicker jutsu.
The Fourth Hokage had not been hokage for very long before sacrificing himself to save the village. Evidence of this can be found in the Kakashi Gaiden series, as Yondaime isn’t wearing his cape, and he wouldn’t be leading his team on a mission if he were hokage at that point already. At that time, Kakashi was about 9-10 years old (Kakashi had stated that "I was younger than Naruto when I became a jonin", and he was not yet a jonin at this point). At the beginning of the Naruto series, Kakashi is about 26 years old, so subtracting 12 (Naruto’s age at that time) means Kyuubi attacked when Kakashi was about 14 years old. So at the very most, the 4th could only have been hokage for 5 years, though it was probably even less than that.
