Do a full backup before you start. Even better, backup the prod, restore it on another server and test your upgrade on that server.
Make sure you read all the upgrade documentation for each version because they contain specificities and special commands that needs to be executed to complete the upgrade.
- https://docs.mongodb.com/manual/release-notes/3.4-upgrade-standalone/
- https://docs.mongodb.com/manual/release-notes/3.6-upgrade-standalone/
- https://docs.mongodb.com/manual/release-notes/4.0-upgrade-standalone/
- https://docs.mongodb.com/manual/release-notes/4.2-upgrade-standalone/
- https://docs.mongodb.com/manual/release-notes/4.4-upgrade-standalone/
For example, this one is common:
db.adminCommand( { setFeatureCompatibilityVersion: "4.2" } )
But there are other useful information in these docs.
From what I’m reading, always make sure to upgrade to the latest minor version for each of the major one: for example, prefer 4.2.20 to 4.2.1, etc.
Cheers,
Maxime.