The easiest way to me seems to be to treat them as 2 different projects.
Just copy the whole “add_on_4” project to the “add_on_5” folder, including the PCB.
Just yesterday I answered a very similar question:
https://forum.kicad.info/t/want-to-start-a-new-version-project-from-existing-customer-one/15392
Some day in the future you may want to reconsider hierarchical sheets.
With hierarchical sheets you can probably make an “add_on_1” schematic, and tell KiCad to replicate that schematic 4 or 5 (or any other number) on the PCB.
With Hierarchical sheets you can also use scripting to replicate the component placement and track layout with scripts.
If you’re interested you can read more about that on:
https://forum.kicad.info/t/replicate-layout-action-plugin/8539
https://forum.kicad.info/t/how-to-repeat-multiple-layout-blocks/14560