This release adds two new experimental features to Nominatim: secondary importance and osm2pgsql flex output.
Importance classification of places is used by Nominatim to properly rank search results and bring the most likely matches on top. The current importance classification based on Wikipedia page rankings is only available for selected objects that have a Wikipedia entry. The new secondary importance is derived from the position of a result and therefore available for all places.
osm2pgsql flex output is a new way of configuring the initial import of OSM data into the place table. The Lua configuration of the flex output will give Nominatim users much more flexibility in the future to control the input.
A more complete list of changes can be found in the Changelog.
Updating from any version between 3.6.x and 4.1.x to the new release is straightforward. Please follow the instructions from the migration guide. Updating from older versions is possible but not recommended.