Database
[{"date":ISODate("2022-10-21T00:00:00.000Z"),task:1},{"date":ISODate("2022-10-20T00:00:00.000Z"),task:2},{"date":ISODate("2022-10-21T00:00:00.000Z"),task:6},{"date":ISODate("2022-10-20T00:00:00.000Z"),task:7},{"date":ISODate("2022-10-20T00:00:00.000Z"),task:8},{"date":ISODate("2022-10-18T00:00:00.000Z"),task:3},{"date":ISODate("2022-10-16T00:00:00.000Z"),task:4}]
Query
db.collection.aggregate([{$group:{_id:"$date",tasks:{$push:"$$ROOT"}}},{$densify:{field:"_id",range:{step:1,unit:"day",bounds:"full"}}},{$addFields:{tasks:{$ifNull:["$tasks",[]]}}},{$sort:{_id:-1},},{$limit:4}])