"When Orochimaru used this jutsu against Hiruzen, three coffins were summoned, but Hiruzen stopped one of them. This third coffin is not seen in the manga, but in the anime it has "four" (四, Yon) written on it. "
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The other coffins had one and two on them, and 1st + 2nd poped out of those. It makes sense since orochimaru wanted to humor 3rd by summoning dead hokages. So either Kishi made a mistake there or souls in dead demon seal are summonable. Imo very slight chance that he wasn't 4th.