Treevet, Thanks for making me go back and study some more. You are correct!!!! Now I have to correct myself in class on Tuesday!! Wall 4 is chemical in nature and as I see it keeps the decay from making a flanking move around the edge of the wound into the wood that forms later. Once this wall is established, then woundwood can cover the outside and seal off the face of the injury from further pathogens.
Here is a link to an image that I took in 1986 of a hackberry that was repeatedly wounded. We can see where many wall 4s were established and breached by the enemy (Leroy and his string trimmer).
http://165.234.175.12/photos/Arbor/mowerslab.jpg