I read at OSNews.com that MySQL 4.1 is now Generally Available (GA). MySQL.com also issued a press release. GA status means the MySQL development team considers the software stable enough for production use. Previously MySQL 4.0 was the GA release and 3.23 was considered "Recent; still supported." Currently both MySQL 4.0 and 4.1 are GA, with 4.1 "recommended" over 4.0. The bleeding edge of MySQL development is 5.0, which is alpha code.
When I began writing Sguil installation docs, I advocated using MySQL 4.0. One of the advantages of MySQL 4.0 is UNION, or the ability to combine the result from many SELECT statements into one result set. While Sguil does not currently use this UNION feature, I expect to see it in future releases. The UNION function in MySQL 4.0 and higher results in much faster queries of session data in Sguil. The OSNews discussion mentions that replication has been enhanced in MySQL 4.1. Replication allows sharing data between two database servers, which is helpful for redundancy in security applications like Sguil.