The Fourth Hokage was the one who sealed the spirit of the Nine Tailed Fox inside of Naruto. In sealing the demon he gave his life and the Third Hokage came out of retirement to take his place. His real name has yet to be mentioned in the Anime, but it was just mentioned in the Manga and it is seen written in Japanese on the summoning contract that Naruto signs to call forth Gamabunta (though it's illegible).

The Fourth Hokage was taught by Jiraiya; the Fourth Hokage was also the one who trained Kakashi who is the sensei of Sasuke, Sakura, and Naruto. It was previously unknown who the teammates of Kakashi were that also trained under the Fourth Hokage, but later it's revealed that they are Obito Uchiha, and Rin.

By his appearance, there was a rumor among the show's fans that the Fourth Hokage could be Naruto's father. Well, not only was this rumor confirmed in chapter 367 of the manga (explained through dialogue by Tsunade and Jiraiya), but also his true name was finally revealed: Minato Namikaze (meaning "port wavewind"). Additionally, we learn that Naruto's mother was Kushina Uzumaki, and that she was a ninja of the Land Of the Whirlpool (Uzumaki is a Japanese word for vortex or whirlpool).

There was also a theory that the Fourth Hokage may be the Akatsuki leader, though not only was this highly unlikely considering that he is supposed to be dead, but it was shown to be false when the true Akatsuki leader, Paine, was revealed. Plus, during the battle between Orochimaru and the Third Hokage, Orochimaru tries to resurrect the three dead Hokages. Three coffins come up, but the Third Hokage forces the last one back... which probably would have been the 4th Hokage.

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 was hokage for only a year or less. 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 13/14 years old. At the beginning of the Naruto series, Kakashi is about 26 years old, which means the Kyuubi attacked basically the year after Obito died.

Apparently "arashi" means "storm".