the kyuubi only increases power and chakra, but it doesnt make you a genius for jutsu's and stuf. and naruto "is" a genius, not everyone can pull of a rasengan, let alone learn kage bunshin in a hour or three. well, all of the "could" be coincidence, but then there are naruto's looks, well.. it "could" be the coincidence again, but hey, would you give something as powerfull as the kyuubi to some newborn baby you don't know? i sure as hell wouldn't. then there is that brother or something from tsunade, he looks the same as naruto. ( although his hair color has been done strange in the anime mind you )
so my theory is, naruto "is" the son of the fourth. if not, it could be his big brother. cuz i remember jiraiya saying he looks like the fourth, and naruto was also the only one being tied to the pole just like yondaime. so that concludes my theory..
as for the thing that the fourth is also in naruto's belly, why didnt naruto see him? another point, i think the jutsu backfired, and that the god of death had something like.. omfg i cant take that kyuubi thingie, yondaime, think of something else will ya? and that is why he took someone else i think
of course this all is just speculation
