Suggestion: Every copper layer is assumed to provide the same completed copper pounds Unless of course if not specified in your fabrication notes. A 4-layer board with a defined pounds of 1oz, is assumed to have a finished copper thickness of 1.37mils (or even more) on each individual copper layer. https://double-sided-tin-sprayed17150.frewwebs.com/31829047/the-basic-principles-of-4-layers-pcb-with-ic-carrier