According to the compatibility matrix at http://docs.mongodb.org/ecosystem/drivers/cpp/ the compat driver should be fully compatible to mongodb 3, but we haven't been able to authenticate using
bool auth(const string &dbname, const string &username, const string &pwd, string& errmsg, bool digestPassword = true);
against a newly created mongo 4 replicaset

This might be caused by tha default authentication change - but I have not been able to find a way to define MONGODB-CR users ?