Upgrading / Upgrading from Bolt 3.0
Bolt 4 is significantly different than Bolt 2 and Bolt 3. As such, there's no simple upgrade. In the near future, this page will have a migration guide, on how to set up a fresh Bolt 4 site, and how to migrate your content and/or theme to Bolt 4.
Until that time, we advise to keep using Bolt 3 for existing websites (it'll keep working for the foreseeable future), and use Bolt 4 for new sites or when you're planning to do a full rebuild anyways.