After three years of development we can finally say that Apache Lucene library and Solr server 4.0 have been released.
Full list of changes in Apache Lucene 4.0, can be found at the following address: http://wiki.apache.org/lucene-java/ReleaseNote40. The full list of changes in Apache Solr 4.0 can be found at the Solr wiki as well: http://wiki.apache.org/solr/ReleaseNote40.
In order to download Apache Lucene 4.0 library please go to: http://www.apache.org/dyn/closer.cgi/lucene/java/. Apache Solr 4.0 can be downloaded at the following address: http://www.apache.org/dyn/closer.cgi/lucene/solr/.
Please note that not all mirrors have to be updated yet.