There is a way to be the Fourth, but very unlikely. In the anime, probably in the manga as well, Kabuto taught Obito Edo Tensei. He got Danzou's bodyguards to show an example. He kills one of them, take his DNA and start the ritual. After it's complete, the dead guy is standing there, but he's not moving. That makes me think that only the body was there, and not the shinobi's soul or mind. As we saw in the Chikara filler arc, Kabuto tried to use some of the Akatsuki members but failed, saying that it was not enough time to use it yet. That could mean that between successive the ritual, there is a cooldown time, maybe the time for his mind or soul take control of the host. Considering this, the most probable thing is that it was the Fourth, but the ritual was made not so far in the past. Orochimaru did complete the ritual, and had Minato, but he couldn't use him right away. He tried to summon him on the fight, but because of the Shiki Fuujin, the host was not taken over by Minato's soul or mind. Either Orochimaru didn't know about the sealing or didn't know about the cooldown time. Minato died protecting the village, but many didn't know how. Orochimaru maybe didn't know he was sealed, and that made the coffin to failed and he being surprised in the fight.