Lightning 3.x vs 4.x

Lightning 3.x and 4.x were, and are, developed and supported in parallel. They both support the same versions of Drupal core: for example, Lightning 3.4.x supports Drupal 8.8 and 8.9, and Lightning 4.1.x does too.

So why are there two major versions of Lightning supporting the same versions of core?

The main difference between Lightning 3 and Lightning 4 is which layout engine they use. Lightning 3.x uses Panelizer for layout; Lightning 4.x uses Layout Builder. That's pretty much it.

This split happened because, at the time that Layout Builder became stable (in Drupal core 8.7, supported by Lightning 4.0), there was no migration path from Panelizer to Layout Builder. However, one was developed after Lightning 4.0 was released, and it's included in Lightning 5.

Therefore, Lightning 5 is a "reunification" -- you can update to it from either 3.4.x or 4.1.x, and migrate your site from Panelizer to Layout Builder at your own pace. Lightning 5 includes Panelizer for compatibility reasons, but uses Layout Builder as its layout engine. Lightning 6 will drop Panelizer entirely.

So when will support for Lightning 3.x and 4.x end? Since both support the last version of Drupal 8, support for them (including security support) will end when Drupal core's support for 8.9 does.