MariaDB - MySQL - ALL-DB Import - Table 'user' already exists
As someone already mentioned, this has to do with the mysql.user table being changed to a view in 10.4. The problem and solution is documented on the MariaDB website in MDEV-22127.
The solution is to simply add the following two lines to the top of your all-dbs.sql dump file:
DROP TABLE IF EXISTS `mysql`.`global_priv`;
DROP VIEW IF EXISTS `mysql`.`user`;