Database
[{"CLIENT_ID":"STK123",x:5,y:1},{"CLIENT_ID":"STK124",x:2,y:7},{"CLIENT_ID":"CP11131",x:1,y:151},{"CLIENT_ID":"LP1WACA",x:3,y:2},{"CLIENT_ID":"XXXX",x:5,y:1},]
Query
db.collection.aggregate([{$match:{CLIENT_ID:{$regex:"^(STK|CP1|LP1WA)",}}},{$group:{_id:{"$substr":["$CLIENT_ID",0,4]},x:{$avg:"$x"},y:{$avg:"$y"},}},{$sort:{_id:1}},{$project:{_id:0,SL:"$_id",x:1,y:1}}])