Three devices authorised - trying to add fourth

You will most likely need to de-authorize all computers, then authorize your 4 computers again. Deauthorize All can only be done once a year.

Authorize or deauthorize your computer for iTunes Store purchases - Apple Support

I am not sure why that is necessary. There is also a limit of 10 devices that can be associated with your Apple ID, including the limit of 5 computers that can be authorized.

View and remove your devices that are associated with Apple ID purchases - Apple Support

Also if your sold or gave away an old computer that had the OS before Catalina, it would need to be deauthorized first. This is where the Deauthorize All would be needed.

What to do before you sell, give away, trade in, or recycle your Mac - Apple Support

gordonfromfoxt wrote:

Thanks for the workaraound - why should that be necessary if I only have three devices? Is Apple addressing this?

This is actually related to this question to which no-one's replied yet

See also Authorization vs. association - Apple Community for some more background. Reformatting an authorized computer without deauthorizing it is a typical way to lose access to one of the five without being aware of it.

Your link doesn't go to a specific post.

tt2