chore(preboot): pin to beta4

Author: MarkPieszak

package.json

@@ -56,7 +56,7 @@
     "moment": "^2.23.0",
     "ngx-bootstrap": "^3.1.4",
     "node-sass": "^4.9.0",
-    "preboot": "^6.0.0-beta.6",
+    "preboot": "^6.0.0-beta.4",
     "raw-loader": "^1.0.0",
     "rimraf": "^2.6.3",
     "rxjs": "^6.3.3",