Weaver Xtreme Plus 4.0 was last updated for WP 5.5, which has not been that long. As a rule we do not release new versions of Weaver Xtreme Plus merely to keep up with the WP compatibility tag. This avoids unnecessary updating of the fairly stable plugin. Version 4.0 has been tested up to the soon to be released WP 5.7.
Weaver Xtreme Plus 3.1.1 is somewhat older. We have not heard any feedback of any incompatibility with WP 5.6, as long as one uses the latest version of Weaver Xtreme. There are in fact known incompatibilities in 3.1.1 with the latest WP versions (jQuery version issues), but because the use of jQuery was coded properly in Weaver Xtreme Plus, it was possible to provide a fix for the problem using standard WordPress features from the theme without needing to update that version of the plugin.