The Machine 4 is one of the Levels in The End is Nigh. It is connected to The Machine 3 from the bottom and The Machine 5 from the right, and "???" areas from the top. 5th ??? area (Left pass) is connected to Retrograde 15.
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Grabbing the ledge at the top right will make it come down, going up will lead to 2 Mega tumors.