This is the fourth release candidate of the Search API Solr Search module, containing some bug fixes and new features.
Most notably, #1846254: Remove the SolrPhpClient dependency removed the dependency on the SolrPhpClient library. (Note: It's therefore important to run update.php immediately after upgrading the module to this version.) This module can therefore now be enabled like any other module, without installing any third-party libraries. After upgrading to this version, you can safely remove the SolrPhpClient library from your server.
#1897386: Update the common schema changed the used configuration files somewhat. You should therefore replace the configuration files and restart the Solr server. The changes are backwards-compatible, though, re-indexing is therefore not necessary (if you are upgrading from RC3 – upgrades from earlier versions still require a re-indexing, as noted in the RC3 release notes).
Also, #1744250: Support location based search added support for location-based searches. For code using this feature, see #1798168: Major module modifications.
If no new major bugs are reported for this release, a stable 1.0 release will be created soon.