4-column grid displaying as 3 columns after WordPress 6.4 update

Symptoms

If you add a Grid block to a page with 4 columns, on the front end these are displayed in 3 columns.

This occurred after the WordPress 6.4 update (and the required update to Blocks 1.6.9).

Workaround

The developers have identified the problem and we should have a hotfix release with a fix soon.