message: docs(aio): Add v4 doc link to navigation
  Fixes
diff: ],
  "docVersions": [
  + { "title": "v4 (LTS)", "url": "https://v4.angular.io" },
  { "title": "v2", "url": "https://v2.angular.io" },
  { "title": "AngularDart", "url": "https://webdev.dartlang.org/angular" }
  