On hard mode of KF, each time an ogre is killed, a knight spawns next to the plate opposite the one the ogre started at.
It is normally considered the responsibility of the one on the plate to ensure the knight is killed, as if it gets to the orb left by the ogre, it will devour it and ruin the run.
Since there is no one on 4th plate, whose responsibility is it to kill the 4th knight?
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By the time the fourth knight comes into play the runner should have the relic and be working on the vessel so the guys who were on the plates should be helping the guys in the middle out by killing the 4th knight so middle two can help kill vessel or any other priority targets.