I think that you will find that the "4-ply rating" applies to the construction of the tyre carcass, not the rubber compound applied to the surface.
I may well be wrong, but I would be surprised if the tyre companies were particularly willing to divulge the precise formulation of their rubber compounds!
Although I suppose a competitor can just buy a tyre and do a chemical analysis...
Why not drop messrs. Dunlop and Michelin a mail with your questions?
SD