As we all know the nine tails fox was sealed away using the Dead Demon Consuming Seal. Once the soul is taken, the Death God devours them both, locking the two souls in an eternal battle in his stomach. However the Fourth Hokage amplified this technique by combining it with the Flying Thunder God Technique. Because of this the nine tails was sealed in Naruto instead of the Death God. So what about the Fourth Hokage? Shouldn't he be there as well? As mentioned previously, the two souls are locked together and placed in an eternal battle in the Death God's Stomach so why can't this happen in Naruto. I believe the Fourth Hokage is inside Naruto but Naruto has not yet explored his inner self. The only room he has ever entered was the one containing the Kyuubi. There has been no inner battle between the fourth Hokage and the Kyuubi because the Kyuubi is locked away.
Later on in the series, either the Fourth Hokage will come to Naruto or Naruto will finally look elsewhere in his subconcious and find the Fourth Hokage or the Fourth will come directly to him. Either way, he is in there.