I just noticed that all the Hokages souls are inside the shinigami that is used for the seal (forgot technique's name)
1st Hokage and 2nd Hokage where sealed away by the 3rd Hokage when he was fighting orochimaru and he summoned them.
3rd Hokage was sealed away by himself trying to kill orochimaru
4th Hokage was sealed away by himself sealing Kyuubi
Time Paradox: If the 4th Hokage sealed himself and was residing in the shinigami stomach for all eternity how could Orochimaru have(attempted to) summon him? even if the 3rd hadn't stopped the coffin of the 4th, would of something appeared since there is no wonder soul of him?