I don’t know if this will help you or not.
Trying to remember upgrading from 4 to 5 I seem to recall that some people had a lot of difficulty with this if they had a lot of terms/categories to deal with that caused it to time out.
Are you using the free version or pro?
What I would try doing is upgrading from 4 to 5.6.2 if using the free version. https://wordpress.org/plugins/advanced-custom-fields/advanced/ and if that worked I would continue to do incremental updates 5.6 => 5.7 => 5.8 => 5.9
I do not see any way to get older version of Pro in order to do this.
Failing that I would contact support and see if they can help.
After this much time, I hate to say it, but you may not be able to get it updated.
