[{"_id":1,"username":"user1","age":1},{"_id":2,"username":"user1","age":1},{"_id":3,"username":"user1","age":1},{"_id":4,"username":"user1","age":1},{"_id":5,"username":"user2","age":1},]

db.collection.aggregate([{$group:{"_id":"$username","count":{$count:{}}}},{$match:{"count":{$gte:4}}}])