Mongo (from "humongous") is a high-performance, open source,
schema-free, document-oriented database. A common name in the
"NOSQL" community.

MongoDB 4.9 is a developer preview. It is not guaranteed to
become an official release. It probably will become the 5.0
version in the future.
See: https://developer.mongodb.com/community/forums/t/mongodb-4-9-on-rpi4-does-not-run/101249/2

WWW: https://docs.mongodb.com/v4.9/