[{"vname":"x","pname":"xy","price":"10000"},{"vname":"x","pname":"xy","price":"10000"},{"vname":"x","pname":"xy","price":"10000"},{"vname":"x","pname":"xy","price":"10000"},{"vname":"x","pname":"xy","price":"10000"},{"vname":"z","pname":"xy","price":"10000"}]

db.collection.aggregate([{$match:{pname:"xy"}},{$sort:{pname:1}},{$group:{_id:"$vname",items:{$push:"$$ROOT"}}},{$project:{_id:0,vname:"$_id",items:{$slice:["$items",4]}}},{$unwind:"$items"},{$replaceRoot:{newRoot:"$items"}}])