Drupal 8.4.0-rc1 is available for testing now. Drupal 8.4 is expected to be released October 4.
8.4.x includes new stable modules for storing date and time ranges, displaying form errors inline and managing workflows. It also includes new stable API modules for discovering layout definitions as well as media management. The media API module is new in core, all other new stable modules were previously experimental. This release also includes several important fixes for content revision data integrity, orphan file management and configuration data ordering among other things.
We have done some initial testing, and have noticed some contributed modules don't play well yet with this release. We had problems with the Superfish module in particular; this module is used on some sites that have dropdown menus. There is an issue in the queue here.