Like Paul said, there are two ways to effect the structure. I haven't
designed, built, or used 4 layer boards in years. Most all 4 layer boards,
in ancient times, were core laminated wherein two double sided "boards" were
laminated together with whatever amount of preg was needed to effect the
required thickness. Mass lamination techniques were, and maybe still, used
extensively using core lamination after printing and etching the inner layer
details.

Most all, if not all, "sophisticated" MLB's are foil laminated with preg
facing the outer most layers with thin core materials making up the rest as
printed and etched details.

Everyone else pointed to oxides and double treat foils. They would impact
you visually as pointed out.

In any event, your boards should be reliable and functional unless your
supplier looses the recipe for effective process management.

Enjoy Genny,

MoonMan

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