database upgrade script failing: Call to undefined method CRM_Upgrade_Incremental_php_FourOne::getMajorMinor()
After some chat on MM we identified that the installation had a file CRM/Upgrade/Incremental/FourOne.php   That file has been removed from the current release but the usual Joomla upgrade process extracts the CiviCRM zip file into the existing code directories rather than removing them first.  This is fine for new and changed files but does not delete files no longer in the released code.
The Joomla upgrade documentation notes that sometimes it is necessary to uninstall and reinstall.  This removes all the old files before installing the new ones.  This should not affect the database but note @minbori's comment above that permissions needed to be reset afterwards. (It would be useful if someone could verify what happens to permissions and add a note to the documentation accordingly.)
Alternatively for those with filesystem access the administrator/components/com_civicrm/civicrm can be deleted and the installation zip file manually extracted to put the new files in place. (See @demerit's comment above.)