Hi Dominique
You can jump from 3.9 to 4.1 in one step, since you are on PHP 7.4.
Warning: There could be other issues. Like rel. 4.1 requires MariaDB 10.4 min. which is higher than the min. for 3.9, which was 10.2.29. Ref. http://www.syndrega.ch/blog/#php-and-dbms-compatibility-of-major-moodle-releases.
P.S. Yours is a different question from the OP Recovery project/Version 1.8.4 php version. I changed the subject accordingly. The moderator might split this part of the discussion.