I think I found a very important hole about edo-tensei, the seal of shikifuujin, and the "revived" hokages:
-Edo-tensei is supposed to "revive" the "dead", not the "sealed".
-Shikifuujin seal is supposed to seal not kill, which means that the Third and the Fourth shouldn't be dead but sealed. The only dead ones should be the 1st and 2nd because they were sealed as edos.
-Releasing the shikifuujin seal should release all 4 hokages as follow:
-1st and 2nd as edos
-3rd and 4th as normal people, not edos.
But in chapters 618, 619 and 620 we clearly see all 4 hokages as edos which means there is a big mistake or a big explanation to be made.
I'm sure Kishi can solve this!!!
4th Hokage was killed by Kurama and 3rd Hokage be Orochimaru