rs.initiate(
   {
      "_id":"rep-set4",
      "members":[
         {
            "_id":0,
            "host":"primary4:27017"
         },
         {
            "_id":1,
            "host":"secondary4_1:27017"
         },
         {
            "_id":2,
            "host":"secondary4_2:27017"
         }
      ]
   }
);