Hello Chris,
Thanks very much for the interest in upgrading.
After purchasing the v4->v8 upgrade ( https://www.hauptwerk.com/hauptwerk-store/ ) you would be sent an iLok activation code for v8. You would then need to follow the steps (for the relevant operating system -- Windows or macOS) in the "Upgrading or re-installing Hauptwerk" section in the main Hauptwerk user guide (pages 26-30 in the current v8.0 version):
https://www.hauptwerk.com/documentation/
If you get stuck or have any questions, do of course just let us know and we can advise.