The upgrade to pro will have the same issues if you have issues upgrading the free version. The developer has done testing and most of the time there has been no issue. Some people do have issues.
If you want to move to pro I would only do that after successfully upgrading to 5.
You should always back up your database before doing the update.
The upgrade instructions are here https://www.advancedcustomfields.com/resources/upgrade-guide-version-5/
There are troubleshooting tips at the end.
If you are still having trouble then you should contact support, links are in the troubleshooting tips, and they will help people that are having issues.
