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

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