Some Stackup presets already exist for manufacturers like JLCPCB or PCBway. AISLER is a european board maker who offers 4/6 Layer boards. It would be great to get the stackups included into flux. Link to the 4 Layer Stackup: https://community.aisler.net/t/4-layer-hd-pcb-stackup/86
(Full disclosure: I am AISLER employee)