This is the fifth release candidate for the Search API Solr Search module. It includes several bug fixes and new features compared to the RC 4 release. If no new bugs are discovered, this will be the last release candidate, followed by the stable 1.0 release.
Most notably, #1676224: Support Apache Solr 4.0 adds support for Solr 4. Note, however, that Solr 4.3 currently isn't supported yet (see #1992806: Fix wrong path to contrib dir in 4.x). There are no known problems with earlier versions of Solr 4, though, and they add vast improvements compared to Solr 3.x.
Also, #1986284: Update common configs to the latest version changed the config files again, which you should therefore update on your Solr server. (Don't forget to restart the Solr server afterwards.) The changes are backwards-compatible, though, re-indexing is not necessary!