The first beta for MJML v4 is here. This version is stable enough so you can start using it.
Install with npm install mjml@next.
You don't need to manually update your templates, they are compatible with MJML v4 out of the box (including this beta). To render your templates, you can:
mjml <yourfile>.mjml -o, the engine will detect if it's using the v3 syntax, migrate it to the v4 syntax, and render it to HTML with the v4 engine. If it's already written in the v4 syntax, it will simply render it to HTMLmjml -m <original-file>.mjml -o <migrated-file>.mjml. This will simply migrate<original-file>.mjmlwritten with the v3 syntax to<migrated-file>.mjmlwritten in the v4 syntax, so you can start using the v4 syntax easily.
To learn about the changelog, breaking changes and new features, read this article (no breaking change since the last alpha).