When upgrading to StackStorm 3.3 where MongoDB 4.0 is the supported version, users should know that the upgrade path from Mongo DB3.4 (prevsiouly supported version) to 4.0 has an intermediate step. The path is:

3.4 -> 3.6 -> 4.0

Following instructions here:
https://docs.mongodb.com/manual/release-notes/3.6-upgrade-standalone/
https://docs.mongodb.com/manual/release-notes/4.0-upgrade-standalone/