[{"_id":1,"price":1},{"_id":2,"price":2},{"_id":3,"price":3},{"_id":4,"price":4},{"_id":5,"price":5}]

db.collection.aggregate([{$setWindowFields:{sortBy:{_id:1},output:{mean:{$avg:"$price",window:{documents:[-4,0]}}}}},{$setWindowFields:{sortBy:{_id:-1},output:{lastMean:{$first:"$mean",window:{documents:["unbounded","current"]}}}}}])