I tend to agree that the 4th is Naruto's dad, but there's one problem with this theory. If the 4th was Naruto's dad, wouldn't everyone know? Cause all the adults know that the kyuubi is sealed in Naruto, if Naruto was truly the son of the 4th, everyone would know that, especially people like the 3rd Hokage, or maybe even Gama Bunta since he was there to fight the Kyuubi. Of course, I suppose it's possible that everyone was instructed not to talk about it, or somehow it was kept a secret. But I'd think that if they did know, then the fact that he's the 4th's son would outweigh the fact that he has the Kyuubi in him. Oh well, just have to wait and see I guess.