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there are 4 zetsus... That got me thinking 1. We know Obito has 3 masks and his last one broke recently... So... maybe three of those zetsus went as a "body shell" for Obito and maybe (if there was more than one Tobi during the Nine tails attack) the last one went with Madara when he was with Itachi... and then after that Madara died later on. That's what i think what happened about the masks i think. Let me know what you think! ^_^