when they showed the 3rd use it, the seal on his belly button appeared like on Naruto. Plus at the first when they show the 4th fighting the Fox Demon, he was on top of Gam... what ever the big frogs name is. And that is the same frog that Naruto can summon and when he first did, Gam. said that he hadn't met anyone like that since the 4th or something like that. And from the series it never says anything about Narutos parents, how he was born, or who at least had him. It would be cool if he was a recarnation of the 4th, then he will defininetly have to become the next hokage.

But the only reason I don't think that his soul is sealed within Naruto is because when *** SPOILER*** Orchi and Hokage battle, Orchi tries to summon up 3 ppl from the dead, but the Hokage stopped the 3th coffin, and the other two was the first 2 hokages, so naturally I would think that the 3rd coffin belonged to the 4th hokage but I could be wrong.