[{"config":{"isEnabled":false,"test":{"sub":[{"sub":[{},{},{},{},{"k":"l"}]},{"sub":[{},{},{},{},{"k":"l"}]},{"sub":[{},{},{},{}]}]}}},]

db.collection.update({"config.isEnabled":false},{"$set":{"config.test.sub.$[].sub.4":{"b":"l"}}})