...Which must be the case because as stated by the Third:
"The 4th sealed the Nine Tails into the belly button on a newborn baby."
Or if you want to use the Viz translation:
"He selected a newborn child, the umbilical cord freshly cut, and bound up all nine tails of the fox within the infant's navel."
That is why the Naruto is the 4th theory makes no sense. Mind you...the whole sealing process doesn't make sense either.

Anyways to get back on topic:
I think at some point the 4th will be a summon or something.