okay so during Naruto's Konoha Crush arc, where Orochimaru and the Hidden Sand/Hidden Sound invade Konoha, Orochimaru and the Third Hokage Hiruzen are on a roof about to fight. Orochimaru reveals his new ability, Summoning: Impure World Reincarnation, which he uses to forcibly summon the first and second Hokage back to life, although admittedly weaker than they were while alive. There's an implication that Orochimaru also attempts to summon the Fourth Hokage as well, but Hiruzen somehow suppresses this; although in the anime all I remember happening is he throws some shuriken at the coffin, so I'm not really sure what that did. Anyways the implication is that Orochimaru had planned to summon the Fourth Hokage using this technique as well, but as we find out later, Minato/Fourth Hokage sealed his soul in the belly of the Grim Reaper when he used the Dead Demon Consuming Seal; Thereby making it impossible for him to be revived using the Impure World Reincarnation Summoning technique. Now of course Orochimaru has never seen this jutsu until Hiruzen uses it to attempt to seal Orochimaru's soul in the belly of the Grim reaper, along with the first and 2nd Hokage's, that being said if it is true Orochimaru was unaware of this jutsu or it's properties, how is Orochimaru able to assumedly summon the 4th Hokage's body? And also, Hiruzen was one of the few people who had a good understanding of the Dead Demon Consuming Seal along with a few choice Konoha shinobi, so why would the 3rd Hokage fear Minato being summoned when he new Minato's soul was sealed in the belly of the Grim Reaper?