I have an unused Cubase 4 ai licence and was wondering if I do upgrade it to C12 ai will I be able to use an older versión in a Macbook Pro with macOS High Sierra?
Why do you want to do that? With a C12 license you can use it on up to 3 different computers.
Just noticed the AI. Since AI is a free version that comes bundled with other products there is no upgrade path TO an AI version. The only upgrade paths are FROM AI to commercially available editions - Elements/Artist/Pro
While an upgrade from AI to Elements 12 probably represents the best value in this case, as the OP is on an older Mac and needs an older version, the Elements 12 license (under the new Steinberg Licensing) won’t allow running previous versions.
Previously, prior to the introduction of the new Steinberg Licensing system, under the old eLicenser system the OP could have purchased a USB-eLicenser, transferred the AI license to that and upgraded it to (at the latest) Cubase Pro/Artist/Elements 11, and would been able to run previous versions on one computer at a time by plugging the USB-eLicenser around.
It’s an unfortunate (and possibly unforeseen) consequence of the combined factors of Apple switching hardware and the introduction of the new Steinberg Licensing system but now, unfortunately, I don’t think there’s any easy way around this, and I would suggest logging a support ticket on MySteinberg, and let the experts in Steinberg advise.