I do believe the theory that the 4th and Naruto are father and son, otherwise where would you get a new born baby from? Do you just walk up to someones house and ask to borrow one for a couple of minutes?
In episode 2 Sarutobi clearly states that the 4th wanted everyone to see Naruto as a hero and that's why he sealed the Kyubi inside of him, plus the Kyubi has a great power that can be used by anyone (otherwise Itachi wouldn't be looking for Naruto), so it would make sence that the 4th would give that power to his son.