There are several reasons why the J! 4 project has stalled: some of these because of technical problems and some of them due to a lack of resources. For some time—for over a year—the most severe of these technical problems went by the label of "release blocker"; that is, a technical issue that fundamentally prevents the release of J! 4 (whether as a candidate for production use or as a suitable platform for stable production use) going forward. It's not that difficult to understand what is a "release blocker" especially in terms of ascertaining understand when or if J! 4 will be released or why there are issues preventing its release.
The last Joomla Bug Squad Team meeting report (dated 12 March 2021) mentions
Fair enough and congratulations to the J! 4 development team for reducing the number of release blockers from twenty-seven three weeks ago to eighteen as of one week ago.Release blockers were the topic of focus for J4. We have 18 open RBs with 7 of those having PRs. We discussed reviewing those and determining next steps with the release lead on what can be moved to 4.1.
If anyone would like to contribute to the release blockers or with the project as a whole please join us on GitHub at https://github.com/joomla/joomla-cms .
I can't locate any items tagged as a "release blocker" today.