In Moodle 4.0, can I publish a message to a course forum (a course that I teach) without logging into my Moodle account, that is, by sending an email to some pre-specified address associated with the forum?
You can reply to a forum post via email but the admin will need to set up the Moodle site to receive incoming emails
https://docs.moodle.org/400/en/Using_Forum#Reply_to_posts_via_email
Then maybe create a pinned forum topic, subscribe your students to it, then reply via email