MongoDB 用 $type 匹配一个数组

我有一个MongoDbCollection ，包含284.116条推文.问题在于，某些对象中的"author"字段是对象类型，但其他对象中的"author"字段是数组类型.所以问题是我想过滤哪些是数组，哪些是对象.

例如:

{
    "_id" : ObjectId("55edfbd11a87d41d987a6dc1"),
    "tweet" : "Back in my dorm, yay!",
    "uri" : "https://twitter.com/natalylug0/status/640994018529181696",
    "date" : "2015-09-08 00:04:17",
    "country" : "U.S.A.",
    "city" : "Texas",
    "state" : "Dallas",
    "author" : {
        "username" : "Nataly",
        "uri" : "https://twitter.com/natalylug0",
        "screenname" : "natalylug0"
    }
}

另一个:

{
    "_id" : ObjectId("55ee3a00e11fbb1030d659fe"),
    "author" : [ 
        {
            "username" : "Relapsed Shini",
            "uri" : "https://twitter.com/iPictoraL",
            "screenname" : "iPictoraL"
        }
    ],
    "tweet" : "@zumbiezuza ???? ily zoeeeeeeee",
    "uri" : "https://twitter.com/iPictoraL/status/641060812140900352",
    "date" : "2015-09-08 01:29:42",
    "country" : "U.S.A.",
    "city" : "Texas",
    "state" : "Dallas"
}

所以我执行了如下查询:

db.getCollection('tweets').find({ author: { $type: 4} })

我得到的是

Fetched 0 record(s) 

但如果执行$type:3，我会得到284.116个值，这和这个集合的大小相同.

所以我的问题是，如何过滤"author"字段包含数组的对象.