It wasn't an unknown jutsu. The Third immediately recognized the jutsu as soon as Oro used it and he knew what was in the coffins before they even opened. So it wasn't an unknown jutsu, it was a forbidden jutsu but the third already knew about it which is why he was able to stop Oro from summoning the fourth. Hell he even knew a third coffin was coming before Oro began summoning it and the technique is obviously pretty old since the hokages themselves knew about it.
I still believe the reason he stopped the third coffin was because he knew that the fourth was in there not because of numbers. He knew we wouldn't be able to stop the fourth but he thought he would have been able to handle the first and the second.
Maybe the coffin that was supposed to contain the fourth would have opened the gateway to hell.
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