Database
[{_id:1,code:"LAZ0101"},{_id:2,code:"LAZ1102"},{_id:3,code:"LAZA0101"},{_id:4,code:"LAZAB102"}]
Query
db.collection.aggregate([{$project:{code:{$substr:["$code",0,4]}}},{"$group":{"_id":"$code","count":{"$sum":1}}},{$project:{count:1,_id:{"$cond":{"if":{$gt:["$count",1]},"then":"$_id","else":{$substr:["$_id",0,3]}},}}},{"$group":{"_id":"$_id","count":{"$sum":"$count"}}},])