I'm Pretty sure about the fourth being Naruto's father.
There are a lot of hints given troughout the series.
For example if I remember correctly Jiraiya apologizes tot the 4th before pushing naruto of a cliff (when he first summons gamabunta)
And Itachi is stronger then orochimaru.
He said it himself, something like : i can't take Itachi's body because he has more strenght as I.
It also seems naruto is cathing up to kakashi because they both had trouble fighting Kabuto (2 unfinished fights).
And I think kakashi is much weaker as orochimaru, the sannin or itachi.
Please correct me if i am wrong on any of this.
I'm sorry.
In the hospital wasn't really a fight.