I don't know about medieval rope customs, all I know about making ropes I learned from googling just now 
For the plant that is subject of this thread, it would be interesting to know if the four ply was used at all in the Middle Ages. Nowadays most reenactors seem to use three ply, but that might just be for practical reasons. It looks like when the separator is shaped like a cross, it's four ply.
The cat o' nine tails would be a cat o' thirteen tails in the VM though.
For the plant that is subject of this thread, it would be interesting to know if the four ply was used at all in the Middle Ages. Nowadays most reenactors seem to use three ply, but that might just be for practical reasons. It looks like when the separator is shaped like a cross, it's four ply.The cat o' nine tails would be a cat o' thirteen tails in the VM though.