Suggestion: Just about every copper layer is assumed to hold the exact same finished copper weight Except normally specified in your fabrication notes. A 4-layer board with a defined pounds of 1oz, is assumed to have a completed copper thickness of one.37mils (or maybe more) on each individual copper layer. https://6-layeruav-specificelectr27159.izrablog.com/31790241/a-simple-key-for-4-layer-communication-pcb-unveiled