Mosaico: Packages are missing (Beta 4)
The "releases" on GitHub don't include the dependencies and aren't supposed to be downloaded directly for "consumption" - the supported workflow is either to use the advanced instructions in the readme file (running install.sh) or to install using the tarball from the CiviCRM extensions directory.

The final build process for mosaico merges GitHub repos to create the tarball.

Extensions Directory page:
https://civicrm.org/extensions/email-template-builder

Chat discussion on the same issue:
https://chat.civicrm.org/civicrm/pl/p7ih3bq36trp5frqnmccy1o3fy