Mongodb double query

I have following MongoDb collection:

 {
     "id":1,
     "isBig": true } {
     "id":2,
     "isBig": true } {
     "id":3,
     "isBig": true } {
     "id":4,
     "isBig": true } {
     "id":5,
     "isBig": false } {
     "id":6,
     "isBig": false } {
     "id":7,
     "isBig": false }

I'd like to choose four random items: 2 should be big and 2 not. My attempt:

db.aggregate([
      { $match: { isBig: true } },
      { $sample: { size: 2 } }
    ]).toArray()

This choose only two big. I would need also two not big. COndition is I can access database only one time.