Recently, I got some of our customers doing an upgrade to v4.x from v3.0 (Yeah, still could see older MongoDB versions and we suggest everyone do the upgrade to stay with the current GA!). There were some pain points in the upgrade process, and especially before migrating to v3.4, you will need to change from [&]

https://www.percona.com/blog/mongodb-config-server-upgrade-from-sccc-to-csrs-replicaset/