Elasticsearch 5.0.0-alpha4, based on Lucene 6.1.0, is the fourth pre-release in the series aimed at testing new features and gathering user feedback before the official 5.0.0 release. This alpha version introduces significant updates, such as the Java REST client, which offers a more stable and flexible HTTP client solution compared to the Java Transport client, and new APIs like Rollover and Shrink for better index management. While the release is not intended for production use, it provides various tools like the Elasticsearch Migration Helper to assist in transitioning from version 2.3.x to 5.0. The release also highlights improvements in search performance, such as the ability to wait for refreshes and support for aggregations in the search profiler. Users are encouraged to download the release, test their applications, and provide feedback on potential issues to help refine the final product.