Communications between members of a replica set are governed by an internal protocol simply referred to as pv0 or pv1. pv0 was the original protocol. Prior to MongoDB 3.2, the only version available was pv0. MongoDB 4.x dropped support for pv0 and only supports pv1. Accordingly, before you perform an upgrade to MongoDB 4, you must reconfigure all replica sets to pv1 (https://docs.mongodb.com/manual/reference/replica-set-protocol-versions/#modify-replica-set-protocol-version).
Fortunately, this process is quite easy, and can be accomplished by this simple procedure. These steps must then be repeated for each replica set: verify oplog entry replication and upgrade to pv1. Let's go into more detail regarding these two steps.
