[{"facilityId":1,"file":1},{"facilityId":1,"file":2},{"facilityId":1,"file":3},{"facilityId":1,"file":4},{"facilityId":1,"file":5},{"facilityId":2,"file":"1"},{"facilityId":2,"file":"2"},{"facilityId":2,"file":"3"},{"facilityId":3,"file":1},{"facilityId":3,"file":2},{"facilityId":3,"file":3},{"facilityId":3,"file":4},{"facilityId":3,"file":5},]

db.collection.aggregate([{$match:{facilityId:{$in:[1,2]}}},{$group:{_id:"$facilityId",files:{$push:"$file"}}},{$project:{files:{$slice:["$files",0,4],}}}])