[{_id:1,content:"a b c d e f g h i j k l m n o p q r s t u v w x y z "},{_id:2,content:"b c d e f g h i j k l m n o p q r s t u v w x y z"},{_id:3,content:"c d e f g h i j k l m n o p q r s t u v w x y z"}]

db.collection.aggregate([{$set:{content:{$reduce:{"input":{$slice:[{$split:["$content"," "]},4]},initialValue:"",in:{$concat:["$$value",{$cond:[{$eq:["$$value",""]},""," "]},"$$this"]}}}}}])