I’ve been using Ver 4 for many years, no longer supported and I’m running into bug issues. Finally broke down and bought Ver 6 but for the life of me cannot figure out a way to export my files from Ver 4 and upload to Ver 6.

Their FAQ has been useless and does not even acknowledge the existence of Ver 4.

Just curious, what makes Goodnotes a better note-taking experience than a simple text editor?

If I were in your situation, I'd look around on my device, or in cloud-connected folders for the data store where GN keeps your notes. It might just be as simple as moving them to another folder or renaming the file(s). Much depends on how GN stores the data, but some form of conversion is probably possible. Note that I didn't say easy, there may be a significant amount of learning and analysis involved.

Alternatively, if you can obtain an older device with GN4 still installable, you could try exporting/importing your notes, or backup/restore, or move to cloud/move back (assuming that GN gives you that functionality, if it doesn't, I'd look for an alternative). I see GN6 is also available for macOS and Windows, which may give you more alternatives.

It is execrable that a software vendor does not provide for retention of user data when offering an "upgrade." Contact support and let them know that you are sharing your experience on social media.